—The community hip protector study is a randomised controlled trial involving women aged 75 years and older who are at high risk of hip fracture and who live in their own homes. Older women living in the northern suburbs of Sydney, Australia, who had contact with an aged care health service and met inclusion criteria were invited to participate in the study. These criteria were age greater than 74 years; two or more falls, or one fall resulting in hospital treatment, in the past year; at least one hip without previous surgery; likely to continue to live in the community for at least three months; likely to survive for at least one year; English speaker; and able to give informed consent.12
A sample of women from the hip protector trial as well as a group of women who had refused to participate in the trial were approached to participate in the quality of life study. The sample included all women randomised into the trial (or who refused to enter the trial) from April 1997 to July 1998. Thus the study elicited values from women who had direct experience in wearing the hip protectors (the intervention group), women who did not have experience in wearing the hip protectors but were aware of the trial (the control group), and women who had refused to participate in the trial because they would not wear the hip protectors if randomised to the intervention group (refusers). The study was approved by the ethics committees of participating hospitals. The quality of life interview schedule was administered to the women six months after they were recruited into the trial (or after refusal to enter).
—To develop descriptions of health states we reviewed the literature and interviewed older women. Sixteen open ended quality of life interviews were conducted with women who had had no contact at all with trial and who had experienced a hip fracture. The interviews helped to define the dimensions of quality of life most affected by a hip fracture and the language used by women to describe their experiences. Data from the qualitative research and clinical opinion were used to generate four “name labelled” health states. The health states were full health (Anne), fear of falling (Mary), a good hip fracture (Jean—where the respondent returns from hospital to independent living in the community), and a bad hip fracture (Elizabeth—where the respondent moves to a nursing home). (See the Appendix
for descriptions of the health states.)
—Respondents were introduced to the purpose of the quality of life study and the format of the interview. Each respondent was asked to rate her own health for each of the five dimensions of Euroqol (EQ-5D) and to assess whether her current health was better, worse, or the same as it was 12 months ago. EQ-5D scores were calculated by using the utility weights of values from a general population survey in the United Kingdom.13,14
In the next stage of the interview, respondents were introduced to the four health states. They were asked to rank the four health states from best to worst. Respondents were then asked to trade off shorter periods of life in full health for longer periods of life with lower quality of life. We used the converging “ping pong” technique to identify their point of indifference.15
We used actual life expectancy as the time horizon for our study. Women aged 75-84 years (most of our study subjects) were given a 10 year time horizon; women aged 85 years and older were given a five year time horizon. To mitigate any ordering effects, the presentation of scenarios was randomly allocated before the interview.
Scoring the time trade off response
—The time trade off technique asks the respondent to choose between two alternatives, both of whose outcomes are known with certainty.14
In this study participants were asked to consider living in a state of less than full health (h<full
) for a defined period of time (t
=5 or 10 years, depending on their age) and then die. The alternative was to live for a shorter period of time in full health (hfull
, represented by the health state “Anne”) and then die. The time (x
) in full health was varied until the subject was indifferent between the two alternatives. The choice scenarios were presented to subjects in six month and one year increments for the five and 10 year interview schedules, respectively. If a respondent would trade off no more than six months or one year (respectively) then they were asked to trade off in smaller increments of one or two months, respectively. The utility weight for each state is given by the formula x
—Power calculation data for comparisons of mean utility scores for independent respondent groups were made by using the guide by Furlong et al.15
We estimated that 70 women in each group would be needed to detect a difference in mean utility scores of 0.1 on the interval scale where α=0.05, power=80%, and SD=0.2 around the mean score. A 10% difference in mean utility score was chosen because it was considered that this represented an important difference in quality of life.
Baseline health assessment
—The general health status and functional capacity of participants was assessed at baseline before randomisation into the trial. The short form-1216
and activities of daily living (Barthel) index8
were administered to each participant in a face to face interview and scored with published scoring algorithms.
Test-retest reliability study
—We readministered the interview schedule to 36 respondents three weeks after their initial interview to assess the reliability of using time trade off in an older population group. The reliability of the utility weights was assessed with the intraclass correlation coefficient.17
Distribution of the time trade off scores—The mean utility weights for both hip fracture states were highly skewed towards zero. Therefore the Mann-Whitney test for comparing two independent samples has been used when appropriate.