When researchers call for a nationally representative sample , they mean that the population of interest is the entire population of the country in question and that the sample should reflect this in its structure.
A national representative sample is a group of citizens that represent the entire country. It is used to carry out research that projects some result that reflects the national reality. It can be preferences of any type, behavior or sociodemographic profiles.
At its best, a representative sample will give the impression of being the entire population, no matter how it looks . The numbers of men vs. women should match national proportions, the percentage in each age group or each region will exactly match the population, etc. For non-demographic measures (such as product ownership or psychographic segmentation) the sample should match the population .
To achieve this, the theory requires a large random sample and a high response rate to minimize systematic error and reduce the risk of unsystematic errors resulting from bias.
You may also be interested in: How to determine sample size .
In the “real world” of online surveys, response rates are not high, and differential response rates by demographic outcome in more purely random samples are not representative of the population (no matter how sophisticated romania phone number stratification techniques are). Therefore, it is typical when conducting surveys to use quota sampling. A quota sample is 100% guaranteed to look like the population of the demographic characteristics you choose to target. However, everything else is subject to sampling error.
Let's take the example of age: if the quotas are set at 16 to 34, 35 to 54, or over 55 the sample will be representative within these proportions, but if the analysis is done in age ranges 16 to 20, 21 to 30, 31 to 40, etc., there is no guarantee that the sample will still be correct.
The extent to which quota control is possible in a sample depends on the sample size and the reference data available in a survey. Six age periods, two genders, and 15 regions create a grid of 180 cells. If the sample size is only 100, it is not possible to fill all the cells. Even with a larger sample size, a cell may require only half a person, and therefore will not have the data in it.
To make a sample more representative, weighting can be used . As an alternative to interleaved cells, quota cells can be structured independently. The disadvantage here is that there can be large “gaps” in the sample – if all the young people are male, for example, it will not be possible to use weighting to correct for the gaps.