People Who Have Known Each Other for a Long Time

People who have known each other for a long time may communicate better than people who have known each other for a short time.Imagine you are carrying out the study suggested in question 6.All your subjects know each other, but for varying lengths of time.How can you make sure that the length of time that subjects have known each other will not be a confounding variable in your study?
You can use the control technique of elimination and only use strangers.However, if you intend to generalize the experimental findings to people who know each other, you need to distribute this subject variable across the treatment conditions instead.You can accomplish this with either a two independent groups design or two matched groups design.