if you are not too bothered erm, if you don't know a priori where the break comes, you can just split the sample size in half and just estimate erm an equation for each, not less, you won't but the computer will, if you just specify half the sample size, right, and when it asks for the number of observations  failure or chow tests you just press the return key and it will use all the remaining observations, right, but when you are doing the empirical work you should always test for structural stability, right, and er either of chow's tests will, will suffice, right, but if you've got  a very small stock sample where there are fewer observations than there are parameters to be estimated you will have to use chow's second test  failure.