How many independent variables should be changed or tested at a time?

Typically, it's best to change or test one independent variable at a time in an experiment. This approach, known as the "one-variable-at-a-time" method, allows for clearer analysis of how that specific variable affects the dependent variable, minimizing confusion from potential interactions between multiple variables. However, in more complex experiments, such as factorial designs, multiple independent variables can be tested simultaneously, but careful consideration and statistical methods are required to analyze the interactions effectively.