The way in which a survey is conducted affects the estimates it produces. The way in which the sample is obtained is arguably the most important factor, particularly for media measurement surveys.

The NRS is based on a random sample of adults aged 15+ in Great Britain.

Only specifically selected individuals can be interviewed to ensure that the sample is as representative as possible – there are no quotas and no substitutions.

People with identical demographic characteristics will have different reading habits according to their lifestyle and availability. NRS uses a random sample to reduce the bias towards people who are more likely to be available for interview than others.

It is difficult to correct for the ‘availability’ bias by weighting. It matters more for certain sorts of publication than others. Hence the importance of ensuring the most representative sample possible.

In order to ensure that the sample is random there are strict rules governing each stage of the decision of whom to interview:

• The areas - based on Output Areas, as defined by the Census
• The specific addresses to approach - drawn from the Postal Address File
• Who should be interviewed at each address - a random selection procedure based on the ages of household members

If you would like to know more about the sampling procedures, please refer to the Technical Information.

Sometimes we are asked why NRS doesn’t use an Internet Panel for the sample, as this would less expensive and more convenient.

It is not possible to obtain an Internet Panel sample which is representative enough to provide a media currency. Online panels tend to be skewed towards particular sorts of respondent, especially heavy Internet users, and cannot represent non-Internet users. While weighting can help correct some of these differences, it is not enough NRS purposes. Indeed, some Internet panels, such as YouGov, use NRS readership data to weight their own findings.

The following chart shows the proportion of the population likely to be represented by different sorts of sample: The NRS sample with a response rate of over 50%; a high quality quota sample; and an online panel.

The response rate shows the proportion of the target sample who were actually interviewed.

It is an indication of quality and how representative the sample is.

Very few surveys provide a full calculation of response rate. Sometimes what is referred to as a response rate is in fact a ‘return’ rate, e.g. the proportion of people approached to complete a panel or postal survey who return a questionnaire.

The NRS response rate is a complete response rate because it reflects both of the following:

• The proportion of the sample that is successfully contacted in the first place (83%)
• The proportion of identified respondents who complete an interview (63%).

Multiplied together, these two elements provide the overall response rate, which was 53% in 2008.

We believe that the single most important factor in maximising the response rate is the use of face-to-face interviewers, who establish a rapport with respondents and encourage their cooperation.

A great deal of effort goes into maximising response rates, e.g.:

• Fieldwork is structured over different days and times of day
• Interviewers make numerous call-backs if necessary
• Explanatory letters and information about the survey are available
• In London and the South East there is a postal follow-up option

NRS response rates have been very consistent over the past five years.

Particular attention has been paid to London, where the response rate has been increased, most notably due the introduction of respondent incentives in 2006.

A more detailed breakdown of the response rate and response rate trends from 1992 to 2009 can be seen here.

NRS recognises that the London area is particularly important to many advertisers and publishers.

London is the most difficult area to interview in – not so much because respondents are unwilling to be interviewed, but because they are less often at home.

Apart from the usual measures of making multiple call-backs, and focusing on evenings and weekends, NRS uses two special measures to increase the response rate in London:

• Since September 2006 London respondents are offered an incentive of £25 in store vouchers for a completed interview. This has increased the London DS-CAPI response rate to 40%
• If a face-to-face interview is not achieved, respondents in London and the South East are sent a self-completion questionnaire in the post

NRS is currently developing a self-completion questionnaire which can be used on the Internet. This will open up new ways to contact and interview London respondents.

A series of weighting procedures are used to ensure that the sample is balanced by sex, age, region, social grade and household size. For details of these procedures, please see the Technical Information.

The sample is grossed to population estimates provided by JICPOPS.