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Whenever I start a new research project, I typically have to find questions and scales that have proven validity in measuring constructs of interest (e.g., psychological / political science / social science scales). This can often be quite time consuming, and often there are issues of cost and copyright to consider.
What are general tips for efficiently and cost effectively locating measures for a given construct?
Are there any databases that make it easy to locate such measures?
Is there anything like a StackOverflow/PatternLibrary/Best Practices compendium of measures or advice on locating measures?
PsychTESTS is a new product by the APA that aims to make it easier to find psychological tests. It is a subscription service. Thus, you would probably have to pay for it yourself or get it through a university or other body that subscribes to it
Read the method sections of the articles that you are incorporating in your introduction and literature review. If you are going to do research you'll need to familiarise yourself with existing research. See what measures have been used in the past.
Ask a question on cogsci.se: Well formulated questions asking for recommendations for measuring particular psychological constructs are welcome on this site.
Measures on a budget
Some tests are free to use. Academics and others often make such tests available on their website (e.g., the IPIP).
For tests developed by academics, you can often just contact the academic by email and ask for permission to use the test. Be clear, be polite, be professional. In some cases they might ask that you share any data from the test with them.
For commercial tests, you may wish to contact the publishers to find out whether they permit free or discounted use for research purposes.
Whether to create your own test
Also as a general point, there is often a decision between whether to buy a test or build your own. Sometimes, the construct being measured is sufficiently novel, that there is no existing measure. However, if the construct does have an established and validated measure, there are strong arguments to use it. Building a well validated psychological test takes substantial time and resources.