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If undercovereage occurs in a sample, it means that
The sample did not cover enough respondents.
The sample size was too small because some subjects did not respond.
The question in the sample was not asked in a thorough enough manner as to capture the true feelings of the respondents.
Certain groups of the population were systematically excluded from the sample.
Calculations in the data results were incomplete and did not give a complete description of the sample.