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1.  Evaluation of a proposed mixture model to specify the distributions of nuchal translucency measurements in antenatal screening for Down's syndrome 
Journal of Medical Screening  2010;17(1):13-18.
A mixture model of crown–rump length (CRL)-dependent and CRL-independent nuchal translucency (NT) measurements has been proposed for antenatal screening for Down's syndrome. We here compare the efficacy of the mixture model method with the standard method, which uses NT multiple of the median (MoM) values in a single distribution.
A routine antenatal screening programme for Down's syndrome comprising 104 affected and 22,284 unaffected pregnancies.
The ability of NT to distinguish between affected and unaffected pregnancies was compared using the mixture model method and the standard MoM method by using published distribution parameters for the mixture model of NT and parameters derived from these for the standard MoM method. The accuracy of the two methods was compared for NT and maternal age by comparing the median estimated risk with the prevalence of Down's syndrome in different categories of estimated risk.
Using NT alone observed estimates of discrimination using the two methods are similar; at a 70% detection rate the false-positive rates were 12% using the mixture model method and 10% using the MoM method. Risk estimation was marginally (but not statistically significantly) more accurate using the standard MoM method.
The mixture model method offers no advantage over the standard MoM method in antenatal screening for Down's syndrome, is more complicated and less generalizable to other data-sets. The standard MoM method remains the method of choice.
PMCID: PMC3104819  PMID: 20356940
2.  Distribution of nuchal translucency in antenatal screening for Down's syndrome 
Journal of Medical Screening  2010;17(1):8-12.
To determine whether the standard deviation of nuchal translucency (NT) measurements has decreased over time and if so to revise the estimate and assess the effect of revising the estimate of the standard deviation on the performance of antenatal screening for Down's syndrome.
Data from a routine antenatal screening programme for Down's syndrome comprising 106 affected and 22,640 unaffected pregnancies.
NT measurements were converted into multiple of the median (MoM) values and standard deviations of log10 MoM values were calculated in affected and unaffected pregnancies. The screening performance of the Combined and Integrated tests (that include NT measurement) were compared using previous and revised estimates of the standard deviation.
The standard deviation of NT in unaffected pregnancies has reduced over time (from 1998 to 2008) (e.g. from 0.1329 to 0.1105 [log10 MoM] at 12–13 completed weeks of pregnancy, reducing the variance by about 30%). This was not observed in affected pregnancies. Compared with results from the serum, urine and ultrasound screening study (SURUSS), use of the revised NT standard deviations in unaffected pregnancies resulted in an approximate 20% decrease in the false-positive rate for a given detection rate; for example, from 2.1% to 1.7% (a 19% reduction) at a 90% detection rate using the Integrated test with first trimester markers measured at 11 completed weeks' gestation and from 4.4% to 3.5% (a 20% reduction) at an 85% detection rate using the Combined test at 11 completed weeks.
The standard deviation of NT has declined over time and using the revised estimates improves the screening performance of tests that incorporate an NT measurement.
PMCID: PMC3104820  PMID: 20356939
3.  UK Lung Screen (UKLS) nodule management protocol: modelling of a single screen randomised controlled trial of low-dose CT screening for lung cancer 
Thorax  2011;66(4):308-313.
The UK Lung Screen (UKLS) is a randomised controlled trial of the use of low-dose multidetector CT for lung cancer screening. It completed the Health Technology Appraisal (HTA)-funded feasibility stage in October 2009 and the pilot UKLS will be initiated in early 2011. The pilot will randomise 4000 subjects to either low-dose CT screening or no screening. The full study, due to start in September 2012, if progression criteria are met, will randomise a further 28 000 subjects from seven centres in the UK. Subjects will be selected if they have sufficient risk of developing lung cancer according to the Liverpool Lung Project risk model. The UKLS employs the ‘Wald Single Screen Design’, which was modelled in the UKLS feasibility study. This paper describes the modelling of nodule management in UKLS by using volumetric analysis with a single initial screen design and follow-up period of 10 years. This modelling has resulted in the development and adoption of the UKLS care pathway, which will be implemented in the planned CT screening trial in the UK.
PMCID: PMC3063456  PMID: 21317179
Health Economist; imaging/CT MRI etc; lung cancer
4.  A strategy to reduce cardiovascular disease by more than 80% 
BMJ : British Medical Journal  2003;326(7404):1419.
Objectives To determine the combination of drugs and vitamins, and their doses, for use in a single daily pill to achieve a large effect in preventing cardiovascular disease with minimal adverse effects. The strategy was to simultaneously reduce four cardiovascular risk factors (low density lipoprotein cholesterol, blood pressure, serum homocysteine, and platelet function) regardless of pretreatment levels.
Design We quantified the efficacy and adverse effects of the proposed formulation from published meta-analyses of randomised trials and cohort studies and a meta-analysis of 15 trials of low dose (50-125 mg/day) aspirin.
Outcome measures Proportional reduction in ischaemic heart disease (IHD) events and strokes; life years gained; and prevalence of adverse effects.
Results The formulation which met our objectives was: a statin (for example, atorvastatin (daily dose 10 mg) or simvastatin (40 mg)); three blood pressure lowering drugs (for example, a thiazide, a β blocker, and an angiotensin converting enzyme inhibitor), each at half standard dose; folic acid (0.8 mg); and aspirin (75 mg). We estimate that the combination (which we call the Polypill) reduces IHD events by 88% (95% confidence interval 84% to 91%) and stroke by 80% (71% to 87%). One third of people taking this pill from age 55 would benefit, gaining on average about 11 years of life free from an IHD event or stroke. Summing the adverse effects of the components observed in randomised trials shows that the Polypill would cause symptoms in 8-15% of people (depending on the precise formulation).
Conclusion The Polypill strategy could largely prevent heart attacks and stroke if taken by everyone aged 55 and older and everyone with existing cardiovascular disease. It would be acceptably safe and with widespread use would have a greater impact on the prevention of disease in the Western world than any other single intervention.
PMCID: PMC162259  PMID: 12829553
6.  Chlamydia pneumoniae infection and mortality from ischaemic heart disease: large prospective study 
BMJ : British Medical Journal  2000;321(7255):204-207.
To determine whether there is an independent association between infection with Chlamydia pneumoniae and ischaemic heart disease.
Prospective study using a nested case-control design.
Medical centre in London run by BUPA, a private medical organisation.
21 520 professional men aged 35-64 who attended for a medical examination in London between 1975 and 1982.
Main outcome measure
Death from ischaemic heart disease.
The distributions of concentrations of IgG and IgA antibodies to C pneumoniae were similar in the 647 men who subsequently died of ischaemic heart disease and in 1294 age matched controls who did not. There was no material association with heart disease irrespective of the cut-off point chosen to define seropositivity. At a cut-off point that defines 15% of controls as positive, for example, the odds ratios were 1.26 (95% confidence interval 0.95 to 1.68) for IgG and 1.09 (0.82 to 1.43) for IgA.
No material association was found between infection with C pneumoniae and ischaemic heart disease. The size and prospective design of the study and the socioeconomic homogeneity of the cohort minimise both random and systematic error.
PMCID: PMC27436  PMID: 10903649
8.  Use of blood pressure lowering drugs in the prevention of cardiovascular disease: meta-analysis of 147 randomised trials in the context of expectations from prospective epidemiological studies 
Objectives To determine the quantitative efficacy of different classes of blood pressure lowering drugs in preventing coronary heart disease (CHD) and stroke, and who should receive treatment.
Design Meta-analysis.
Data source Medline (1966-2007).
Study selection Randomised trials of blood pressure lowering drugs recording CHD events and strokes. 108 trials studied differences in blood pressure between study drug and placebo (or control group not receiving the study drug) (“blood pressure difference trials”), and 46 trials compared drugs (“drug comparison trials”). Seven trials with three randomised groups fell into both categories. The results were interpreted in the context of those expected from the largest published meta-analysis of cohort studies, totalling 958 000 people.
Participants 464 000 people defined into three mutually exclusive categories: participants with no history of vascular disease, a history of CHD, or a history of stroke.
Results In the blood pressure difference trials β blockers had a special effect over and above that due to blood pressure reduction in preventing recurrent CHD events in people with a history of CHD: risk reduction 29% (95% confidence interval 22% to 34%) compared with 15% (11% to 19%) in trials of other drugs. The extra effect was limited to a few years after myocardial infarction, with a risk reduction of 31% compared with 13% in people with CHD with no recent infarct (P=0.04). In the other blood pressure difference trials (excluding CHD events in trials of β blockers in people with CHD), there was a 22% reduction in CHD events (17% to 27%) and a 41% (33% to 48%) reduction in stroke for a blood pressure reduction of 10 mm Hg systolic or 5 mm Hg diastolic, similar to the reductions of 25% (CHD) and 36% (stroke) expected for the same difference in blood pressure from the cohort study meta-analysis, indicating that the benefit is explained by blood pressure reduction itself. The five main classes of blood pressure lowering drugs (thiazides, β blockers, angiotensin converting enzyme inhibitors, angiotensin receptor blockers, and calcium channel blockers) were similarly effective (within a few percentage points) in preventing CHD events and strokes, with the exception that calcium channel blockers had a greater preventive effect on stroke (relative risk 0.92, 95% confidence interval 0.85 to 0.98). The percentage reductions in CHD events and stroke were similar in people with and without cardiovascular disease and regardless of blood pressure before treatment (down to 110 mm Hg systolic and 70 mm Hg diastolic). Combining our results with those from two other studies (the meta-analyses of blood pressure cohort studies and of trials determining the blood pressure lowering effects of drugs according to dose) showed that in people aged 60-69 with a diastolic blood pressure before treatment of 90 mm Hg, three drugs at half standard dose in combination reduced the risk of CHD by an estimated 46% and of stroke by 62%; one drug at standard dose had about half this effect. The present meta-analysis also showed that drugs other than calcium channel blockers (with the exception of non-cardioselective β blockers) reduced the incidence of heart failure by 24% (19% to 28%) and calcium channel blockers by 19% (6% to 31%).
Conclusions With the exception of the extra protective effect of β blockers given shortly after a myocardial infarction and the minor additional effect of calcium channel blockers in preventing stroke, all the classes of blood pressure lowering drugs have a similar effect in reducing CHD events and stroke for a given reduction in blood pressure so excluding material pleiotropic effects. The proportional reduction in cardiovascular disease events was the same or similar regardless of pretreatment blood pressure and the presence or absence of existing cardiovascular disease. Guidelines on the use of blood pressure lowering drugs can be simplified so that drugs are offered to people with all levels of blood pressure. Our results indicate the importance of lowering blood pressure in everyone over a certain age, rather than measuring it in everyone and treating it in some.
PMCID: PMC2684577  PMID: 19454737
10.  Serum vitamin E and subsequent risk of cancer. 
British Journal of Cancer  1987;56(1):69-72.
In a prospective study of about 22,000 men attending a screening centre, serum samples were collected and stored. The concentration of vitamin E (alpha-tocopherol) was measured in the stored serum samples from 271 men subsequently notified as having cancer and from 533 unaffected controls, matched for age, smoking history and duration of storage of the serum samples. The mean vitamin E level of the cancer subjects was not significantly different from that of their matched controls. The mean level in the cancer subjects who were diagnosed as having cancer before the elapse of one year from the date of blood collection was, however, significantly lower than the mean concentration of their matched controls (10.0 and 11.5 mgl-1 respectively, P = 0.003). For subjects whose cancers were diagnosed one or more years after blood collection the difference was not statistically significant either for all cancers or for cancers of six sites considered separately, viz. lung, colon and rectum, stomach, bladder, central nervous system and skin. The most likely explanation for these results is that the low vitamin E levels observed in these subjects were a metabolic consequence, rather than a precursor, of the cancer. This would explain, at least in part, the overall inverse association between serum vitamin E and risk of cancer observed in the published epidemiological studies on serum vitamin E and cancer.
PMCID: PMC2001682  PMID: 3620319
11.  Plasma retinol, beta-carotene and vitamin E levels in relation to the future risk of breast cancer. 
British Journal of Cancer  1984;49(3):321-324.
In a prospective study of 5,004 women in Guernsey, plasma samples were collected and stored. Retinol, beta-carotene and vitamin E levels were later measured in the samples from 39 women who subsequently developed breast cancer and from 78 controls who did not develop cancer. Plasma retinol levels were not related to the risk of breast cancer, mean levels among cases and controls being 485 micrograms l-1 and 479 micrograms l-1 respectively. Plasma vitamin E levels showed a clear association, low levels being associated with a significantly higher risk of cancer. The mean vitamin E levels among cases and controls were 4.7 mg l-1 and 6.0 mg l-1 respectively (P less than 0.025), and the risk of breast cancer in women with vitamin E levels in the lowest quintile was about 5-times higher than the risk for women with levels in the highest quintile (P less than 0.01). beta-carotene levels showed a tendency to be lower in women who developed cancer than in controls (36 micrograms l-1 among cases compared with 50 micrograms l-1 among controls) but the difference was not statistically significant.
PMCID: PMC1976754  PMID: 6704307
13.  Death from ischaemic heart disease. 
British Medical Journal  1977;2(6089):772-773.
PMCID: PMC1632050  PMID: 912309
15.  Insulin-like growth factors and cancer: no role in screening. Evidence from the BUPA study and meta-analysis of prospective epidemiological studies 
British Journal of Cancer  2006;95(1):112-117.
Insulin-like growth factor-1 (IGF-1), insulin-like growth factor-2 (IGF-2), and insulin-like growth factor binding protein-3 (IGFBP-3) were measured in frozen serum samples from 1051 men with cancer and 3142 controls in a nested case–control study from the British United Provident Association (BUPA) study cohort and associations with 14 cancers were examined, including prostate, colorectal, and lung. A meta-analysis of studies on these three cancer sites was also conducted. In the meta-analysis the odds ratio between the highest quartile IGF-1 group and the lowest quartile group was 1.31 (95% confidence interval (CI): 1.03–1.67) for prostate, 1.37 (1.05–1.78) for colorectal and 1.02 (0.80–1.31) for lung cancer, and for IGF-2 it was 0.72 (0.36–1.44) for prostate and 1.95 (1.26–3.00) for colorectal cancer. Results from the BUPA study were consistent with the estimates from the other studies. There were no statistically significant associations with IGFBP-3 and any of the cancer sites considered. Our results suggest that IGF-1, IGF-2, and IGFBP-3 measurements have no value in cancer screening, although IGF-1 and IGF-2 may be of aetiological significance in relation to colorectal and prostate cancer.
PMCID: PMC2360494  PMID: 16804529
cancer screening; meta-analysis; cohort studies; insulin-like growth factors; epidemiology
16.  The tar reduction study: randomised trial of the effect of cigarette tar yield reduction on compensatory smoking. 
Thorax  1995;50(10):1038-1043.
BACKGROUND--Observational and short term intervention studies have reported that smokers of low tar cigarettes inhale more deeply (that is, compensate) than those who smoke high tar cigarettes. To quantify this effect a long term randomised trial was conducted on the effects of switching to low tar cigarettes. METHODS--The trial was carried out between April 1985 and March 1988 among cigarette smokers in the British Civil Service, measuring blood carboxyhaemoglobin (COHb) levels and serum cotinine levels as markers of tobacco smoke intake. Volunteers first switched to a cigarette brand yielding around 10% less tar than their usual brand to identify smokers able to change brand. The 434 subjects who successfully switched were then randomly allocated to one of three groups: (a) "fast reduction" group which changed to a brand of cigarettes with a tar yield of about half that of their usual brand; (b) "slow reduction" group which reduced to the same level in steps over several months; and (c) a control group which continued smoking cigarettes with a tar yield 10% lower than their usual brand. RESULTS--Over the course of the trial cigarette consumption declined slightly in all three groups. In both the "fast reduction" and the "slow reduction" groups, intake of COHb and cotinine was reduced, though not to the same extent as the yield reduction. Comparison of the results before randomisation with those at the end of the trial showed that a reduction in carbon monoxide yield of 45% was associated with a decrease in carbon monoxide intake of 19% (95% confidence interval 14% to 24%) and that a reduction in nicotine yield of 40% was associated with an 11% (6% to 16%) reduction in nicotine intake, reflecting relative intakes of about 1.5 for both carbon monoxide and nicotine in the "fast reduction" group. Results were similar in the "slow reduction" group with a 42% reduction in carbon monoxide yield, a 16% (11% to 22%) reduction in carbon monoxide intake, a 37% reduction in nicotine yield, and a 6% (0% to 13%) reduction in nicotine intake. Estimates of compensation derived from these results were 65% for carbon monoxide, 79% for nicotine, and 62% for tar. CONCLUSIONS--Compensation, demonstrated when switching from a high tar cigarette to a low tar one, was incomplete. Advising people who have failed to give up smoking to switch to low tar cigarettes will reduce the intake of smoke constituents to a small extent. This would be expected to decrease their risk of smoking-related diseases, although by a smaller amount than would be achieved by giving up smoking altogether.
PMCID: PMC475015  PMID: 7491550
17.  Occupational exposure to hydrazine and subsequent risk of cancer. 
OBJECTIVES--The aim was to examine the cause specific mortality of men exposed to hydrazine. METHODS--Hydrazine was produced at a factory in the east midlands between 1945 and 1971. The cohort of all 427 men who were employed there for at least six months with varying degrees of occupational exposure to hydrazine were followed up until the end of January 1992. RESULTS--By the end of July 1982 49 deaths had occurred and the observed mortality was found to be close to that expected at each level of exposure. By the end of January 1992 a further 37 deaths had occurred. Again the observed mortality was close to that expected for all causes and also for lung cancer, cancers of the digestive system, other cancers, and all other causes, irrespective of the level of exposure. CONCLUSIONS--The results weigh against there having been any material hazard of occupational exposure to hydrazine. The small number of men studied means, however, that a relative risk as high as 3.5 for lung cancer cannot confidently be excluded.
PMCID: PMC1128148  PMID: 7697139
18.  Disagreements are not substantial. 
BMJ : British Medical Journal  1994;308(6935):1027-1029.
PMCID: PMC2539874  PMID: 8167518
19.  Systematic underestimation of association between serum cholesterol concentration and ischaemic heart disease in observational studies: data from the BUPA study. 
BMJ : British Medical Journal  1994;308(6925):363-366.
OBJECTIVE--To estimate the size of the association between serum concentration of low density lipoprotein cholesterol and mortality from ischaemic heart disease. DESIGN--Prospective study of total serum cholesterol concentration and mortality from ischaemic heart disease in 21,515 men (538 deaths) and study of total cholesterol concentration measured on two occasions an average of three years apart in 5696 men in whom low density lipoprotein cholesterol concentration was also measured on the second occasion. SUBJECTS--Men who attended the medical centre of the British United Provident Association (BUPA) in London between 1975 and 1982. MAIN OUTCOME MEASURE--The difference in mortality from ischaemic heart disease for a 0.6 mmol/l difference in concentration of low density lipoprotein cholesterol after adjustment for, firstly, regression dilution bias, which arises from the random fluctuation of serum cholesterol concentration in people over time, and, secondly, the surrogate dilution effect, which arises because differences in total cholesterol concentration between people reflect smaller differences in low density lipoprotein cholesterol concentration. RESULTS--The observed difference in mortality from ischaemic heart disease associated with a difference of 0.6 mmol/l in total serum cholesterol concentration was 17% but increased to 24% after correction for the regression dilution bias and to 27% (95% confidence interval 21% to 33%) after adjustment for both sources of underestimation, which provides an estimate of the difference in mortality for a true difference of 0.6 mmol/l in low density lipoprotein cholesterol concentration. The association was greater at younger ages. The estimated decrease in mortality from all causes was 6% before and 10% (1% to 17%) after adjustment for the two sources of underestimation. There was no excess mortality from any cause associated with low cholesterol concentration. CONCLUSIONS--The association between serum cholesterol concentration and ischaemic heart disease is materially stronger than directly inferred from prospective studies. This has important implications for the health benefit of achieving low cholesterol concentrations.
PMCID: PMC2539480  PMID: 8124143
20.  Assessing possible hazards of reducing serum cholesterol. 
BMJ : British Medical Journal  1994;308(6925):373-379.
OBJECTIVE--To assess whether low serum cholesterol concentration increases mortality from any cause. DESIGN--Systematic review of published data on mortality from causes other than ischaemic heart disease derived from the 10 largest cohort studies, two international studies, and 28 randomised trials, supplemented by unpublished data on causes of death obtained when necessary. MAIN OUTCOME MEASURES--Excess cause specific mortality associated with low or lowered serum cholesterol concentration. RESULTS--The only cause of death attributable to low serum cholesterol concentration was haemorrhagic stroke. The excess risk was associated only with concentrations below about 5 mmol/l (relative risk 1.9, 95% confidence interval 1.4 to 2.5), affecting about 6% of people in Western populations. For noncirculatory causes of death there was a pronounced difference between cohort studies of employed men, likely to be healthy at recruitment, and cohort studies of subjects in community settings, necessarily including some with existing disease. The employed cohorts showed no excess mortality. The community cohorts showed associations between low cholesterol concentration and lung cancer, haemopoietic cancers, suicide, chronic bronchitis, and chronic liver and bowel disease; these were most satisfactorily explained by early disease or by factors that cause the disease lowering serum cholesterol concentration (depression causes suicide and lowers cholesterol concentration, for example). In the randomised trials nine deaths (from a total of 687 deaths not due to ischaemic heart disease in treated subjects) were attributed to known adverse effects of the specific treatments, but otherwise there was no evidence of an increased mortality from any cause arising from reduction in cholesterol concentration. CONCLUSIONS--There is no evidence that low or reduced serum cholesterol concentration increases mortality from any cause other than haemorrhagic stroke. This risk affects only those people with a very low concentration and even in these will be outweighed by the benefits from the low risk of ischaemic heart disease.
PMCID: PMC2539477  PMID: 8124144
21.  By how much and how quickly does reduction in serum cholesterol concentration lower risk of ischaemic heart disease? 
BMJ : British Medical Journal  1994;308(6925):367-372.
OBJECTIVE--To estimate by how much and how quickly a given reduction in serum cholesterol concentration will reduce the risk of ischaemic heart disease. DESIGN--Data on the incidence of ischaemic heart disease and serum cholesterol concentration were analysed from 10 prospective (cohort) studies, three international studies in different communities, and 28 randomised controlled trials (with mortality data analysed according to allocated treatment to ensure the avoidance of bias). MAIN OUTCOME MEASURE--Decrease in incidence of ischaemic heart disease or mortality for a 0.6 mmol/l (about 10%) decrease in serum cholesterol concentration. RESULTS--For men results from the cohort studies showed that a decrease of serum cholesterol concentration of 0.6 mmol/l (about 10%) was associated with a decrease in incidence of ischaemic heart disease of 54% at age 40 years, 39% at age 50, 27% at 60, 20% at 70, and 19% at 80. The combined estimate from the three international studies (for ages 55-64 years) was 38% (95% confidence interval 33% to 42%), somewhat greater than the cohort study estimate of 27%. The reductions in incidence of ischaemic heart disease in the randomised trials (for ages 55-64 years) were 7% (0 to 14%) in the first two years, 22% (15% to 28%) from 2.1-5 years, and 25% (15% to 35%) after five years, the last estimate being close to the estimate of 27% for the long term reduction from the cohort studies. The data for women are limited but indicate a similar effect. CONCLUSIONS--The results from the cohort studies, international comparisons, and clinical trials are remarkably consistent. The cohort studies, based on half a million men and 18,000 ischaemic heart disease events, estimate that a long term reduction in serum cholesterol concentration of 0.6 mmol/l (10%), which can be achieved by moderate dietary change, lowers the risk of ischaemic heart disease by 50% at age 40, falling to 20% at age 70. The randomised trials, based on 45,000 men and 4000 ischaemic heart disease events show that the full effect of the reduction in risk is achieved by five years.
PMCID: PMC2539460  PMID: 8043072
22.  Relation of urinary cotinine concentrations to cigarette smoking and to exposure to other people's smoke. 
Thorax  1990;45(5):356-361.
The relation of urinary cotinine measurements to tobacco consumption in smokers and to exposure to other people's smoke in non-smokers was studied in 49 smokers and 184 reported non-smokers attending a health screening centre. The median urinary cotinine concentration was 1623 ng/ml in the smokers and 6.1 ng/ml in the non-smokers. In smokers the average urinary cotinine concentration increased with reported habitual cigarette consumption; in non-smokers it increased with the reported total seven day duration of exposure to other people's tobacco smoke. Cotinine concentrations were approximately three times higher in non-smokers living with a spouse or partner who was a smoker than in those living with a non-smoker; their reported duration of exposure to tobacco smoke was also three times higher. Non-smoking subjects who were exposed to any tobacco smoke and who lived with a smoker reported 70% of their exposure to be at home (56% for men and 86% for women); the men reported more exposure at work than non-smoking men who lived with a non-smoker. This study confirms the relation of urinary cotinine to stated tobacco smoke exposure in both smokers and non-smokers and further validates the use of information on the smoking habits of the spouse or partner as a measure of tobacco smoke exposure in epidemiological studies of non-smokers.
PMCID: PMC462471  PMID: 2382242
25.  By how much does dietary salt reduction lower blood pressure? II--Analysis of observational data within populations. 
BMJ : British Medical Journal  1991;302(6780):815-818.
OBJECTIVE--To determine whether the estimates of the size of the association between blood pressure and sodium intake derived from studies of individuals within populations can be quantitatively reconciled with our estimates derived from comparisons of the average blood pressure and sodium intake between different populations. DESIGN--Examination of data from 14 published studies that correlated blood pressure recordings in individuals against measurements of their 24 hour sodium intake (within population studies). MAIN OUTCOME MEASURE--Comparison of observed differences in blood pressure per 100 mmol/24 h difference in sodium intake in each within population study with predicted differences calculated from the between population data, after allowing for the underestimation of the true association of blood pressure with sodium intake caused by the large day to day variation in 24 hour sodium intake within individuals. RESULTS--The underestimation bias inherent in the within populations studies reduced the regression slope of blood pressure on single measures of 24 hour sodium intake to between a half and a quarter of the true value (for example, in one study from 6.0 to 2.4 mm Hg/100 mmol/24 h). Estimates from between population comparisons of the regression slope of blood pressure on sodium intake, after adjustment to take this underestimation bias into account, were similar to the values actually observed in the within population studies. CONCLUSION--The within population studies confirm our estimates from between population comparisons of the magnitude of the association between blood pressure and sodium intake.
PMCID: PMC1669173  PMID: 2025704

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