Well, if your sample does not represent the larger sample, you'll certainly not get a valid result ...
For example, if you're studying pregnancy and your sample includes men -
The whole idea of "representative' sample is fuzzy and often gives interesting errors.
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Because without representative sample, your results will not be valid.
A sample that accurately reflects the characteristics of the population as a whole
Take a simple random sample.
A representative sample is one where the statistics of the sample are the same as the statistics for the parent population.
It is important for a sample to be representative, so that the whole population's view is counted, and not biased by one particular group within the population.