Academic journal article Population

Attrition in the COCON Cohort between 2000 and 2002

Academic journal article Population

Attrition in the COCON Cohort between 2000 and 2002

Article excerpt

To improve knowledge of contraceptive use and induced abortion in France, a team composed of researchers from INSERM, INED and CNRS initiated a cohort study - the COhort CONtraception or "COCON" survey - in 2000. The main aims were to analyse social factors linked to the occurrence of unplanned pregnancies and requests for abortion, and to study the responses of the health system and the effects of the different contraceptive methods on women's health. The survey methods used (described in detail in the introductory article) were organized around two major decisions: first, to re-interview the participating women each year over five years (between 2000 and 2004); second, to conduct the interviews by telephone.

All longitudinal studies are confronted with the problem of sample attrition or loss to follow-up, i.e. the loss of part of the sample between successive interview waves. Geographic mobility and refusals to answer second or subsequent interviews mean that it is practically impossible to contact all of the subjects present at one wave in the next wave. Yet unequal probabilities of re-interview according to individual characteristics can bias analyses made at the cohort level. Furthermore, attrition raises the question of loss of precision in the estimates obtained, since this loss increases as sample size falls.

The COCON cohort did not escape the phenomenon of attrition. During the first round of interviews (2000), 2,863 women were questioned, but those re-interviewed in 2001 and 2002 numbered only 2,218 and 1,912 respectively, corresponding to a one-third reduction between the first and third waves. It is thus important to understand the reasons for this loss to follow-up and to evaluate its implications for the analyses. In particular, we want to test whether attrition causes bias in the variables of interest to the survey. In the course of this evaluation, we consider whether telephone interviews at regular intervals are an appropriate instrument for follow-up surveys on such a sensitive topic.

After describing the data and techniques used, we present the results of the analysis: first, the magnitude of the attrition and its determinants, second, the analysis of the biases.

I. Method of analysis

This section presents the data used to characterize the attrition and sets out our method of analysis. Unless otherwise stated, all the variables included in the analysis relate to characteristics reported by the women during the first wave of interviews (2000).

Unless otherwise stated, the analyses use unweighted data. We considered the sample for the first wave as a complete population subject to attrition, which meant that we attributed the same weight to each of its units, independently of how it had been selected. The results of the analysis justified this choice a posteriori because the main variable determining the weighting (the stratum to which sample members belong, based on experience of unplanned pregnancy or induced abortion) was virtually unrelated to attrition.

1. Descriptive analysis

The descriptive analysis of attrition is based primarily on the "contact files", which record details of all calls made to the women. The data provide information about the outcome of each call: refusal, person unknown, absent, appointment arranged, etc.

We used these "contact files" to define the following criteria for each woman that we wanted to re-interview:

- non-contact: when the interviewers never managed to speak with the woman during the data collection period (whether or not contact was made with another member of the household);

- refusal, which took two forms: 1) refusal expressed at the end of the previous year's interview (explicitly or because the woman did not want to leave a number where she could be contacted); 2) refusal to participate when contacted the following year, including explicit refusals but also women who made appointments for interviews that never actually took place within the collection period, which can be treated as a disguised form of refusal. …

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