Preventing a user from reentering an already visited page

Even if have we have excluded access to a WWW client's menu, as described in the previous section, a user can often, e.g. with the help of a right mouse click in many clients, access a small menu that at least allows to go back to an already visited page. We can hardly prevent a user from doing this, but we can make sure that the user can not gain anything from such an endeavour.

If you have access to the client, disable caching completely. This will require to reload previous pages.

Further note that the state of the experiment should always be stored on the server and never on the client. Assume you would store relevant information, like a participant's budget in a trading experiment only on the client. A participant could spend the complete budget on a transaction and sent this information to the server. The server verifies the transaction, finds that the budget is suffient, implements the transaction and sends a page back to the client. While all this is done, the participant might retransmit the page and, thus, carry out another transaction -- with a budget that would otherwise allow only a single transaction. The sever had to accept the second transaction, since it arrives again with the first budget information. This can not happen if all relevant information, here the budget, is stored on the server.

Oliver Kirchkamp 2013-04-15