These reasearchers are doing a study to see how playing different types of music can influence brain activity in a select group of people. Also recording emotions they feel.
Lets begin by meeting our participants!
The researchers find that a vast majority of their randomly selected participants are female! This is unintentional and surprising to them...
Oh no... this is a case of selection bias because the unintentional, large difference in gender/sex in the group can create a different outcome by influencing the independent variables! This wasn't meant to be a study on mainly women...
The effects of music reported by the group could be swayed... How on earth can we fix and this?
Yes , that would control the gender/sex variable in our group and now allow it to interfere with the results! You're a genius Charlie!!
read first: I've got it! Next time, We can fill but limit the randomized selections to be 25 females and 25 males!!