Valid results are only obtained if each tube is identical because; effective comparison of the tested blood each time can only be achieved if the testing model is constant in which case each tube is identical.
Discussion:
During scientific tests such as that described in the question; it is important to keep all parameters of testing constant.
This is so because, the utmost motive of the testing model is to draw comparison. It is on this note that valid results are only obtained if each tube is identical.
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