B2 2.7 How valid is the data?

Investigations about distribution of organisms in their environment can be very difficult. That is because they are often done over a long period of time and not all variables can be controlled.
If a transect is made at the beach during the morning, a comparative investigation must be done at the same time of day even if it is two months later. The time o day is a control variable. We can control this variable. In a valid investigation alll possible variables must be…

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