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Some subjects may try to participant in the same study more than once
To overcome this problem, you can ask for the email addresses of each participant, and then look for duplicates.
Since nowadays its easy for people to create multiple email addresses, you can also ask for name and/or address of each subject. Sometimes researchers will have a "lottery" as incentive to participate (e.g., $100 lottery prize for each 400 participants), so asking for name/address is necessary to award the lottery check.
You can also collect the IP address of each participant and look for duplicates. One issue here is that sometimes DSL providers give the same IP address to multiple people.
Some subjects may drop out of the study before finishing
In traditional laboratory-based research its unusual for a subject to walk out of a study, but online a subject can get distracted or simply lose interest and end the study. Sometimes researchers will have a "lottery" as incentive to have the subject participate in the study, but with any type of monetary incentive IRB's typically require a statement in the consent form saying something to the effect of "you may discontinue participation at any time without any consequences or losing your entry in the lottery."
Since a certain number of online subjects wont finish the study, you can overcollect the number of subjects you think you need to offset the number of subjects who dont finish the study, usually around 10-20%.
Some subjects may stop the study and then continue minutes/hours later
The problem here is that some studies involve manipulations which may lose power if there is a time lag between the manipulation and measures in the study. One advantage of online studies is that you can record how long the subject is taking part in the study, so you can identify the average length of time of your study, and also identify those subjects who take an extrordinary long amount of time to finish the study.
Some subjects may rush through the study
Since you can record how long each subject is participating in the study, you can identify those subjects who may have rushed through the study.
How do I get subjects for my online study?
There is no single answer to this question and you may have to try a number of things before you find something that works for your study, depending on the subject matter and incentives you are offering. See How to recruit subjects online? for an ongoing discussion of subject recruitment.

As human enters a age of information technology.The advantages and disadvantages becomes increasingly prominate.The advantages are that internet made life much more convenient and effcient.People can ...