Sample Bias in Study I have following Study statement: 
A council wishes to study the digital awareness of its resident senior population (over 65 years), so it questioned in person 50 residents randomly chosen from a database of the entire resident population, collecting, among others, the number of digital equipment to which it has access. 
What is sample bias in this,from the following options:
•   The population database
•   It's not skewed
•   Population born in the county
•   Age 
•   Be a resident of the county and belong to the database 
•   Several, including number of digital equipment that has access.
I personally feel the sample bias could be because of age or Database. 
But age is ruled out because the study is all about senior residents therefore age factor wouldn't be compromised in the sampling and cannot lead to sample bias. 
On the other hand I can presume that not all senior residents are in Database. Therefore, second last option is valid. As senior residents who are not part of database are left out. 
Kindly share your opinions so that I can be clear of this conceptual confusion.

Snoke,
Sampling bias are things that would cause the surveyors to under-sample or over-sample a subset of a population.
I think you could make an argument for or against most of these options given enough context. Access to digital equipment maybe pertinent if it skews who has can participate in the study. Age or national origin could be impacted by sampling bias for instance if the surveyors did not go to nursing homes or policing discouraged immigrant participation. 'Belonging to the database' might make sense as well if the non-database sub-population had different characteristics than the database sub-population.
