When collecting feedback from customers it is important to clarify whether you want feedback from every customer (this is a "census" of all customers), or whether you only want feedback from a sample of your customers.  Most often, departments only collect information from a sample of customers - it is cheaper, easier and quicker. 
 
If you are getting feedback from a sample of customers it is important to ensure the sample is representative of all customers. If you have an unrepresentative (or biased) sample of customers you will get unrepresentative (biased) responses. 

For example, feedback based on comments from customer complaints systems will probably be representative of dissatisfied customers, but is highly unlikely to give an accurate picture of the views of satisfied customers.

In other words, as much as is possible, make sure you get feedback from a mix of customers - men, women, different ages, different socio-economic and cultural backgrounds, urban and rural residents etc. - so your results will be truly indicative of the experiences of all your customers.