When the other key variables are not controlled in an investigation we cannot draw a conclusion about the effect of one variable in an investigation because if we can’t control the other key variables, you will not be able to tell which variable is responsible for the observed results.
For example – if two key variables change at a time in an experiment say temperature and humidity, you cannot be sure which of these causes the change in the dependent variable, it may be temperature or it may be humidity or it may be a combination of both.