Health and Care Experience Survey 2023/24 Technical Report

Sample Design

Sampling Frame

Public Health Scotland (PHS) selected a random sample of people who were registered with a General Practice in Scotland, had an address in Scotland and were aged 17 and over. PHS co-ordinated checks against the NHS Central Registry and the Community Health Index (CHI) database to remove people from the sample who have passed away.

People registered with some general practices run by NHS Boards to provide primary care services to particular groups of people (e.g. practices for homeless people, armed force or for prisoners) were excluded from the survey due to low response rates from these groups, the sensitivities that contacting them would require and the difficulties involved in contacting these groups by post.

A number of practices that had been excluded from previous surveys due to low response rates were included in the 2023/24 survey. These included a number of practices associated with universities that historically had a low response rate.

Sampling Design and Sample Size Calculation

The sample was stratified by General Practice, with the aim of collecting a sufficient number of responses to be representative of the patients’ experience at each practice.

For some practices with very small numbers of eligible people, all patients were included in the survey in order to meet the minimum sample size requirements identified.

Minimum number of responses (M)

For each General Practice, we calculated sample sizes to reach the Minimum number of responses (M) required to achieve an estimate of a percentage that has a 95 per cent confidence interval with width +/- eight percentage points, sampled from a finite population.

The formula for the minimum number of responses required (M) is

M = B / (1+((B-1) / N))

Where:

• B = z²p(1-p) / c² = 150 using the following definitions:
  • p is the proportion answering in a certain way, assume 0.5 to give maximum variability;
  • z is 1.96 for a 95 per cent confidence interval (using the standard normal distribution);
  • c gives maximum acceptable size of confidence interval, in this case 0.08 (eight percentage points).
• N is the number of people on the sampling frame (i.e. the number of people registered with a practice and aged 17 or over);

Table 1 shows the minimum number of responses required (M) based on the assumptions above, for some example practice population sizes.

An illustration of the number of responses required to achieve the desired level of accuracy by the practice list size.

In practice, if the proportion who answering a particular way is actually higher or lower than 0.5, then these numbers of responses would give narrower confidence intervals (or fewer responses would be required for the same accuracy).

The minimum number of responses required is adjusted upwards to allow for assumed non responses to the survey.

Estimated response rates to the 2023/24 survey for each individual General Practice were based on the response rate of the 2021/22 survey. Where response rates were not available i.e. for a new practice assumed response rates were used based on the proportion of the eligible population living in the most deprived 15% of data zones (based on the Scottish Index of Multiple Deprivation 2020), which affect the likelihood of a person responding to the survey. Estimated required sample sizes were capped at a maximum of 1,000 for individual practices.

The addresses from CHI were cross checked against the Scottish Postcode Directory to ensure that they were complete. Any instances of invalid, deleted or incomplete postcodes were removed prior to sample selection, as were a small number of people who had requested not to be included in this or other surveys.

A total of 528,376 people were sampled for inclusion in the Health and Care Experience Survey 2023/24.

PHS checked for any cases where the same name (first name, middle name and surname) and address appear. Each of these cases within the “duplicate” are removed prior to sample selection. The duplicates may be relatives living together or errors in the source data. Removing them ensures there is no ambiguity as to who is being asked to participate in the survey and reduces the risk of questionnaires being sent out in error.

Sample Selection

For the majority of practices in Scotland, a random sample of the required number of people from each practice was taken from the CHI database using the sampling frame by Public Health Scotland. For some practices with very small numbers of eligible people, all were included in the survey in order to meet the minimum sample size requirements identified from the calculation above. The sample was selected using the statistical software package R.