Olfactory function was assessed in all respondents using tests of both odor sensitivity (i.e., the lowest concentration at which an odor can be detected) and odor identification. Odorants were administered using commercially available felt-tip pens, each filled with an individual odorant at a specific concentration. This device is inexpensive, convenient, and ideally suited to delivering odorants at a constant concentration (Hummel, Sekinger, Wolf, Pauli, & Kobal, 1997
). After we developed the NSHAP protocol, another short screening method using the pens was independently developed (Mueller & Renner, 2006
The test of odor sensitivity involved presenting a series of five pens, the first containing only the diluent propylene glycol (1,2-propanediol) followed by steadily increasing concentrations of the odorant n-butanol (0.13%, 0.50%, 2.00%, and 8.00%). Each pen was held by the interviewer (who wore a cotton glove to eliminate residual odors) approximately half an inch from the respondent’s nostrils; respondents were then asked to inhale through the nose, during which time the pen was waved slowly back and forth for no more than 3–4 s. Following each pen, respondents used a visual analog scale ranging from 0 (labeled no smell at all) to 10 (labeled smells very strong) to record their perception of odor strength.
Although there is not sufficient space to present the sensitivity data here, we note that there was some variation among respondents in the way in which the visual analog scale was completed. Respondents were given a choice between recording their responses directly on the interviewer’s laptop computer (using the mouse to position a slider) or on a paper version of the scale. Nearly two thirds of those who completed the olfactory module chose the paper version (61%), and the likelihood of choosing paper was higher for women and increased with age. Despite instructions to mark an X on the line representing the scale, 17%–18% of those who recorded their answers on paper wrote an integer between 0 and 10 instead.
Following the sensitivity test, respondents were presented with a five-item identification test. A single odor was presented, and respondents were asked to identify it from a set of alternatives (responses were recorded by the interviewer on the computer). This was repeated using five individual odors. The response sets were as follows (in order of administration and with the true odorant indicated in italics): (a) chamomile, raspberry, rose, or cherry; (b) smoke, glue, leather, or grass; (c) orange, blueberry, strawberry, or onion; (d) bread, fish, cheese, or ham; and (e) chive, peppermint, pine, or onion. Following a forced-choice paradigm, respondents were not permitted to answer “don’t know”; however, for each test, 1%–3% of respondents refused to answer. Although these refusals are excluded from the analyses presented here, they may in many cases reflect uncertainty about the correct response, and therefore, other analysts may wish to handle them differently.
Results for the identification tests are presented in . For this analysis, we have focused solely on whether the respondent was able to identify the odorant correctly; a more in-depth analysis might examine the distribution of responses among the various incorrect alternatives. Each of the five odorants was identified correctly by a majority of respondents, with peppermint identified correctly most often (92%) and leather identified correctly least often (71%). Item nonresponse (including item-specific refusal to give a response plus 61 respondents [2%] who declined the entire olfactory module, 1 respondent who broke off the interview at an earlier point, and one instance of an equipment problem) was highest for the first item (5%) and declined steadily thereafter. The increasing likelihood of a correct response coupled with decreasing item nonresponse is consistent with the possibility that some respondents became more adept at the task after the first couple of tries. However, because the order of the items was identical for all respondents, it is not possible to distinguish between such order effects and true item-specific differences in difficulty.
Item-Response Models Fit to Odor Identification Data (SEs)
Estimates for Model 1 and two versions of Model 2 (see Multiple Measurements
) are also presented in , obtained using all respondents for whom data from at least one item were available. Model 1 reflects the same ordering in item difficulty observed in the percent correct and estimates the variance of the αi
to be 1.27 on the logit scale, indicating that a 1 SD
increase in individual ability roughly triples the odds of correctly identifying a given odor. Model 2A permits the effect of a change in individual ability to vary across items; a likelihood ratio test of this model against Model 1 yields a p
value of <.001, indicating that the items do differ in their ability to discriminate among individuals. Estimates of the discrimination parameters
indicate that peppermint provided the best discrimination, whereas rose and leather provided the worst.
Model 2B extends 2A by incorporating a structural model for the αi
containing the covariates gender and age group. The estimated odds ratio for women (vs. men) is e0.32
= 1.38, with an approximate 95% confidence interval of 1.20–1.58. The effects of age appear roughly linear, with a 67% decrease in the odds of identifying an odor correctly from the youngest (57–64 years) to the oldest (75–85 years) age group (odds ratio 0.33 with a 95% confidence interval of 0.26–0.43). These results are consistent with previous studies of gender and age differences in olfactory function assessed by odor identification (Hummel et al., 2007