Implementation of the FSA Listeriosis Guidance: Chapter 8: Technical appendix on IFF survey

Sampling

The HSC (non-NHS Trust) sample for the study was drawn from four sources: 

• Market Location provided settings in the social care category. This included nursing homes, day centres for the elderly/vulnerable, residential care homes, community meal provision (e.g., ‘meals on wheels’) and assisted living developments for the elderly. 
• Wilmington Healthcare supplied IFF Research with settings in the healthcare category; this consisted of hospices, private hospitals and day procedure units.
• As Market Location did not have enough community meal provision in their database to reach our quota target in this category, desk research was conducted in order to ‘top-up’ the sample. 
• IFF Research also utilised snowball sampling, asking those who completed the survey if they could give us contact details of the commercial meal providers they use. However, during fieldwork it became clear that very few HSC (non-NHS Trust) settings use outside caterers, with just 14 settings (3%) agreeing this was the case. With a number of interviewees providing details of the same meal provider, only a small fraction of the interviews in the study (two out of 445 completed interviews) are from commercial meal providers.

The sample was stratified by country and setting type to broadly reflect the underlying population, though some categories were overrepresented (such as healthcare settings and community meal provision) to give more robust base sizes for analysis. 

Questionnaire design

The questionnaire for HSC (non-NHS Trust) settings was designed by IFF Research and the FSA to best meet the research objectives.  It took the NHS survey as a basis, maintaining comparability where possible, but making refinements and additions where it was felt that this would collect more comprehensive or accurate data.  The performance of the questionnaire was monitored during the early part of the fieldwork period and a few small improvements made. 

Weighting

We used the amount of sample available from Market Location and Wilmington Healthcare in relevant categories as the most comprehensive available source of population data.  We then adjusted these figures to exclude the proportion of each sector which had been found to be out of scope of the survey either because the settings did not sell chilled ready-to-eat food or because the settings did not ever cater for vulnerable groups. 

The tables below show the proportions of each setting type we excluded from the scope of the research and the final unweighted and weighted proportions for each setting type (sample definition), and by country. 

As a note, we gave HSC (non-NHS Trust) settings the opportunity to tell us what kind of setting they are as part of the research. While we have used their self-definition for analysis purposes, the weighting was done based on the definition of their setting which was marked up on the sample records, as this felt the most directly comparable to the population data.

Table 8.1 Proportion of each setting that were excluded after being found to be out of scope

Table 8.2. Unweighted and weighted proportions of completed interviews

Country

Response rates

A total of 3,761 HSC (non-NHS Trust) setting records were eligible to be contacted over the course of the survey. As listed in Table 8.1, 359 sites were found to be out of scope due to not having ready-to-eat food available or because they did not provide food to vulnerable groups. A further 2,634 records were out of scope for fieldwork. For example, 162 had unobtainable numbers or the site was closed. Table 8.2 breaks down those who could not be reached during the fieldwork period and are thus not included in response rate calculations, since no firm contact was made. 

This left 768 records in scope for fieldwork, of which 445 completed an interview. This equates to a response rate of 58%, as shown in Table 8.4.

Table 8.3. Setting outcomes of the total sample in scope of study

Table 8.4. Sample outcomes of the total sample in scope of fieldwork