I am conducting a diff-in-diff regression over two periods of time with the treatment in between. Next to the dummy variable time ('Post'), I have 3 groups for the other variable and therefore 2 dummy variables ('app', 'instagram'). I want to investigate the exposure to news media over different channels over two periods of time.
My question now is: - What is the adequate process to finding out the relevant covariates which I shall include as control variables?(e.g. age/gender/graduation/media use behavior etc)
- And how do I include them methodically?
I tried proceeding with a crosstabulation of the covariates to the DV and looking at the eta values. I chose this procedure because I read it is better if the potential covariates are nominal. I built dummy variables for every expression, e.g. media use behavior often, media use behavior sometimes, media use behavior never). Is it right that I made binary dummy variables for a categorical variable like this (which I questioned via 5-point-likert scale? Do i just include the variables with an eta-value over 0.3 as covariates? (I thought this could be a good threshold correlation-wise)
Is there some analysis afterwards necessary (I read of Manova, but I am very unsure if sth like this is necessary)
I am sorry for the long text, but I am really looking for help. I am doing this for the first time ...