Gulf War Veterans Measuring Health (1999) / Chapter Skim
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B: Design Issues in the Gulf War Veterans Health Study
B: Design Issues in the Gulf War Veterans Health Study
Pages 96-126
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From page 96... ...
, it is important for the study to provide timely information of value to the GW veterans during their lifetime. Thus, the study should be designed with more frequent data collection in the early years when the information obtained has a longer "useful life." Based on the consideration of various trade-offs, three of the most promising designs are the permanent panel design, the repeated panel design, and a combination of the two.
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From page 97... ...
The membership in this population was determined by the participation in the Persian Gulf War, that is, those who served in the Gulf War theater of operations between August 2, 1990, and June 13, 1991. The closed nature of the GW veteran population has important implications for the study design, such as on the merits of replenishing the panels Further discussions are given in the section on temporal structure below.
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From page 98... ...
In population studies of Gulf War veterans conducted to date, response rates ranged from a low of 31% in the study conducted by Stretch et al.
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From page 99... ...
based on the actual cost data obtained in the field. In order to facilitate He recruitment of the second panel if warranted, it is worth considering recruiting a "reserve" sample along win the initial panel, giving Hem a brief enrollment interview to collect contact information, and maintaining contact with them over time through tracking.
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From page 100... ...
4. Repeated panel surveys with temporal overlap.
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From page 101... ...
5. Repeated panel surveys without temporal overlap.
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From page 102... ...
The uncertainty In the estimated net change includes bow Me sampling error in the cross-sectional samples, it, and Me temporal random error, it. Win more overlap, such as In panel surveys, the same individuals are observed at times O and 1, Bus the sampling error ~ is cancelled when we compare times O and 1, resulting in a more precise estimate for Me net change.5 For estimating levels, the overlap is usually a disadvantage.6 More specifically, consider the estimation of the average level of Y across the population and also over time.
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From page 103... ...
However, since the anticipated between-wave lags are much longer than the recall period for GWVHS, this feature is unlikely to be applicable. Further discussions on bounding the time frame and related recall error issues can be found in the section, Measurement Error.
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From page 104... ...
Therefore the representativeness of the panel becomes compromised in subsequent waves, both because of the omission of new members, and because of the cumulative attrition discussed earlier. Some panel studies refresh the sample by adding a sample of new members who joined the target population since the original sample was drawn.
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From page 105... ...
The permanent panel design has the advantage that it allows the direct assessment of long-term gross changes on the same individuals with the repeated panel surveys without temporal overlap, we need to "splice" the trajectory from different panels to assess changes across waves that fall under different panels. However, the permanent panel design will be more vulnerable to cumulative attrition and panel conditioning.
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From page 106... ...
However, it does allow for the estimation of gross changes in later waves, say, between the third and fourth waves. As discussed in He Introduction to this appendix, the public health focus of the GWVHS indicates that it is more important to assess changes across the first three waves (repeated panel surveys without temporal overlap is preferable for those objectives)
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From page 107... ...
The survey frequency is then determined by the number of waves to be conducted within the given duration. For many panel studies, the survey frequency is determined by the reference periods for the key outcome measures (see the section, Measurement Error below)
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From page 108... ...
Those techniques can be used in conjunction with information about survey costs to compare alternative survey frequencies to determine the appropriate design that provides the optimal power and precision under the available budget. In addition, one also needs to take into consideration attrition and measurement error issues discussed in the section Nonresponse, Attrition, Tracking, and Tracing and in Measurement Error, in order to arrive at the appropriate choice for the survey frequency.
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From page 109... ...
Therefore hmeliness of ~nforrnation should be taken into consideration for the design of the GWVHS. In basic science research, we usually assume an infinite time horizon.~3 The scientific knowledge acquired from the study is expected to sense a large number of fixture clients in the years to come.
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From page 110... ...
Timeliness of information (the value of the interim data) is usually not taken into consideration for study designs because the dissemination phase subsequent to the trial has an infinite time horizon.
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From page 111... ...
The portion of the reduction available after the conclusion of the study is shaded in dark gray. Since the study period is short compared to the dissemination period, the value of the interim data is low compared to the value of the final study results available after the conclusion of the study.
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From page 112... ...
Further research in this area would be worthwhile, not only to address the design issues in the GWVHS, but also to address similar public-health-oriented issues. SURVEY MODALITY AND GEOGRAPHICAL CLUSTERING With the revolution in communication and information technologies in recent years, there are many modalities to be considered for conducting a survey study, including face-to-face interview, telephone interview, mail administration, and combinations of those modalities.
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From page 113... ...
In order to maximize the comparability between the GWVHS sample and the non-GW veterans sample with medically unexplained physical symptoms, it would be desirable to take a portion of the GWVHS sample from the same geographical locales where the non-GW veteran cases with medically unexplained physical symptoms are sampled. In addition, if physical examinations will be conducted on a subsample of GWVHS participants, it would be worth considering making the subsample geographically clustered, to facilitate the face-toface contact for the physical examination.
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From page 114... ...
To a limited extent, baseline nonresponse bias can be mitigated using nonresponse weighting and posts/ratification weighting, (multiple) imputation, pattern mixture modeling, and selectivity modeling (see, e.g., Brick and Kalton, 1996; Copas and Farewell, 1998; Heckman, 1979; Little, 1993, 1994; Little and Rubin, 1987; Rubin, 1987, 1996; and Schafer, 1997~.
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From page 115... ...
This is one of He main reasons many panel studies use the repeated panel design to regain the cross-sectional representativeness. To a limited extent, wave nonresponse bias can also be mitigated using the techniques discussed earlier for baseline nonresponse (see, e.g., Copas and Farewell, 1998; Diggle, 1989; Diggle and Kenward, 1994; Diggle et al., 1994; Hec~nan and Robb, 1989; Kalton, 1986; Kyriazidou, 1997; Laird, 1988; and Lepkowski, 1989~.
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From page 116... ...
A variety of public information sources are usually used for tracing, such as telephone directories, credit records, property records, court records, mortality records, (to identify deceased sampled subjects)
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From page 117... ...
Some types of measurement errors are specific to panel surveys. We discuss both types of measurement errors, and remedies that can help mitigate problems resulting from those measurement errors (see, e.g., Bailar, 1989; Groves, 1989; Groves and Couper, 1998; Kish, 1965, Chapter 13; and Lessler and Kalsbeek, 1992~.
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From page 118... ...
However, random error might result in the overestimation for individual level gross changes. There are many techniques and procedures that can be used to mitigate measurement error in the survey data.
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From page 119... ...
is used, in which the respondent's response is randomized to help alleviate his concerns. For survey questions that inquire about the respondent's past experience or future anticipation, the level of measurement error is determined by the reference period (see below)
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From page 120... ...
.'9 The nature of the reference period for a specific survey measure has important implications on the measurement error (see, e.g., Bailar, 1989; Neter and Waksberg, 1964~. The respondent might misclassify the time for specific events relative to the reference period, resulting in telescoping (inclusion of events that occurred outside the reference period)
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From page 121... ...
Measurement Error Issues Specific to Panel Surveys One of the advantages in using panel surveys is that the previous interview and/or events reported during the previous interview can be used as milestones to bound the reference period, to help improve the accuracy of the respondent's recall. This advantage is applicable when the reference period coincides with the lag between successive waves of surveys.
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From page 122... ...
It is much more difficult to assess panel conditioning with either the permanent panel design or repeated panel surveys without temporal overlap. The comparison across waves cannot be used to assess panel conditioning because it is confounded with true changes over time.
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From page 123... ...
1989. Information Needs, Surveys, and Measurement Errors.
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From page 124... ...
1986. Handling Wave Nonresponse in Panel Surveys.
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From page 125... ...
1989. Treatment of Wave Nonresponse in Panel Surveys.
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From page 126... ...
In: Panel Surveys (Eds.
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