Scientific method calls for experimentation where you vary one variable at a time, observe the effect, making further changes only after consideration of the cause and effect relationships in the first experiment are understood.

Unfortunately, this is the opposite approach unwittingly adopted by many improvement initiatives, where there is a brainstorming session to identify “improvement opportunities” which are listed, prioritised, and implemented.

In the event of any improvement happening, we cannot tell which of the changed variables drove it, indeed, you may have good ideas in the mix whose positive  impact is masked by the poor ideas and their outcomes.

One at a time takes more time, but not only offers the certainty of a positive outcome, it also educates you on the reasons why improvement has occurred, which can only benefit the ongoing process.