Problem of Missing Data
From PsychWiki - A Collaborative Psychology Wiki
Why is missing data a problem? Missing values means reduced sample size and loss of data. - You conduct research to measure empirical reality so missing values thwart the purpose of research. The less data collected, the less data that can be analyzed.
Missing values may also indicate bias in the data. - If the missing values are non-random, then the study is not accurately measuring the intended constructs. The results of your study may have been different if the missing data was not missing. See Why do missing values occur? for the difference between random and non-random missing data.