1. Is the sample representative of the Population ?  
 Is the
"sampling
frame" representative?  Is the sample biased? Statistical bias
is broader than personal
bias.
A very common bias is "nonresponse bias." 

2. Is the sample large enough?  
Sample Size for Forecasting a Close Election  
3. Do the questions tempt respondents toward biased ansuers?  
4. What is the agenda of the people collecting and presenting the data?  
5. Do the respondents know the answers to the questions?  
6. Is there a standard to compare the results against?  
Weakness of beforeandafter  
7. What are the alternative explanations for the results?  
No Explanation
(Random Chance) Transient Explanation Other Explanations 

8. Is there a statistical "lie" in the presentation of the results?  
Note on How to Lie with Statistics 