Hi

Validation of survey instruments should preferably be done against the very thing you want to measure. So, if you want to validate a questionnaire which is supposed to measure school aptitude, you will need to correlate it with some sort of objective measure of this variable, probably test results and/or grades. This is criterion validity, and if you look up this term in some book on social science methodology, you will find a wealth of information. See for example Anastasi 1988 Psychological testing.

So, a pilot study can be enough to validate an instrument, it depends upon how you do it. However, repeated validations in different populations is preferable.

Thanks Wahlberg