Are there any books or journals I should read that may provide information about how long an intervention should last in a quantitative study?

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Simone Cheli

For the sample size you may use a free calculator such as:

http://biostat.mc.vanderbilt.edu/wiki/Main/PowerSampleSize

http://www.stat.ubc.ca/~rollin/stats/ssize/b2.html

For the duration, remember that the statistical significance is due to both the sample size and the number of observations. So you need to better understand what kind of analysis you want to perform. I suggest to read books as the ones below:

https://books.google.it/books?id=ur0BlXPuOukC&printsec=frontcover&dq=longitudinal+analysis&hl=it&sa=X&ei=Nj8DVe_dNareywOhwILgAQ&redir_esc=y#v=onepage&q=longitudinal%20analysis&f=false

https://books.google.it/books?id=0exUN1yFBHEC&printsec=frontcover&dq=longitudinal+analysis&hl=it&sa=X&ei=HD8DVYzZD-vnygOxl4GoDg&redir_esc=y#v=onepage&q=longitudinal%20analysis&f=false

Mehdi Osooli

Dear Iris,

There are many resources on sample size calculation for various study designs. I recommend you to consult a biostatistician for this as Simone already has introduced some of them. But specially about how long one should keep an intervention, its a question which is very much related to the nature of intervention and lag between the intervention and appearance of its results.

The point here is basically how long you expect some outcomes of interest happen after your intervention. But keep in mind also that some outcomes may change or disappear after some time therefore you should be careful for not only getting enough sample size and observations but also ensuring that you have achieved stable outcomes.

Good luck.

Mehdi

Vuyisile Nkonki

Though there are resources on sample size calculations which you could carry out, the statistical power of that sample size is even more important. May I suggest that you also read around statistical power. Depending on whether you are measuring immediate effects or long term impact of the intervention, the length of time lapse after the intervention will have to be taken into consideration. So, simmering, stabilisation, and sustainability would be your other consideration if you are looking at the time lag for measuring post-intervention.