How to compare regression models for two different data sets? looking for some further help and to further my understanding of regression topic area. 
So I am trying to compare regression models with two different data sets A and B. 
Using dataset A, I fitted the regression model y=  mx1+ nx2 + c this gave rqs =95% 
I want to know how well this model works for dataset B. I have tried to understand this using two methods. 


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*Using the same variables x1 and x2, fit regression model for dataset B which gives me y=ox1+px2=c (different coefficients and intercept) with r sq = 75% 

*I have used the regression eq obtained in original fit y=  mx1+ nx2 + c and input the x1 and x2 from dataset B to obtain predicted values for y. I have then plotted the real y values against the predicted y values and assessed the r sq value. 
Is any of these methods valid? Or is there a better method for comparing regression models? I have some initial limitations with using method 2 as the model is fitted the real values to a predicted value from a prediction. Can anyone help my understanding of this? 
 A: https://en.wikipedia.org/wiki/Chow_test discusses the CHOW TEST which can be used to test the hypothesis of a common set of parameters over k groups ( 2 in your case ). I routinely use this in a time series context to DETECT the point in time when the model parameters change significantly
A: There is a logical gap between the two methods, if we want to be precise. The first method aims to assess whether the same variables significantly describe the data of dataset 2. While the second method assesses the out-of-sample performance of the equation estimated on the first dataset (I.e the ability to use the equation estimated in dataset 1 to predict the dependent variable in the dataset 2). Clearly there is a logical difference, because in the first method you are just looking at the common variables (with any coefficient), while in the second you are testing the feasibility of using the exact relationship estimated in dataset 1 to interpret dataset 2. So it depends on what your purpose is.. maybe the two datasets share the same relevant variables but with very different coefficients (in this case you may have the first method saying the variables are the same, because the same variables are significant in both the dataset, but their “true coefficients” vary significantly across the two datasets so the second method gives disappointing results because the first equation performs poorly in terms of fit in the second database).. 
So if your objective is to predict dataset 2 based on the info in dataset 1 (as I believe reading your problem description), then choose the second method.. instead, if you want to assess whether the two common variables are significant in both the dataset (regardless the shape of the relationship and the coefficients), then choose the method 1 and test whether one/both the variables are significant for the model in dataset 1 and 2 (regardless the coefficients)
I also endorse the solution by @IrishStat (upvoted as such), to test whether the parameters of the relationship change across the two samples
