"Students on Campus" According to the man, why is it important to mention that the subjects were randomly selected?

Narrator: Listen to part of a conversation on campus between two students.
Woman: This is an interesting assignment.
Man: It is. I’m just having a problem figuring out how to write up the report.  Q18
Woman: Oh? How much have you done?
Man: Well, the introduCtiOn was easy, Ijust expaned on the nformation that the lab   Q19
    assistant provided, you know, about the effects of alcohol on reflexes, and I used the same
    references that he cited.
Woman: Me, too.
Man: Then I described the experiment in the second part... the methods and materials
    section. Woman:What did you include there? The lab assistant said that it was important to be specific
    when we did this part.
Man: Yeah, he did. So I mentioned that there were ten subjects, and five were drinking gin
    and tonic, but the other five... that’s the control group... they were drinking tonic only,
    and no one knew which group was which. Then they each had to drive in a
    computerized... what was it called? A... a simulator... a simulator of a car that was supposed
    to be moving at 35 miles an hour. And when they saw a bicycle, they were supposed to
    hit the brakes.
Woman: So far so good. The only thing I can think of that you might want to add is maybe a little
    more about the subjects. I think they were college students, and they were selected at
    random to be in the experimental or control group.
Man: Good idea. Especially the part about the random selection, That would be imporant  Q20
    information if someone wanted to duplicate the experiment, and didn’t he say that this
    section had to be specific enough for another researcher to be able to repli...repli...
Woman: replicate.
Man: Yeah. Replicate the study.
Woman: Okay, so what did you do with the results section?
Man: That’s the problem. I can’t seem to figure out what to put in the results section and what
    to use for the discussion section. If I put the chart with the reaction times in the results,
    should I explain the chart in the discussion, or what?
Woman: Oh now I see the problem. The charts fairly self-explanatory, isn’t it? I mean, it’s really  Q21
    a simple chart to read.
Man: That’s what I thought.
Woman: But you still have to explain the chart in the results section.
Man: You do?
Woman: Yeah. Um, look, here’s what I’d do. I’d look at the chart and try to come up with a few
    general statements. For example, um well, all of the subjects who were drinking  Q19
    alcohl had longer reaction times. Then you can look at the range--1 second to 4 seconds
    for the subjects who were drinking only tonic, and that’s compared with 3 seconds to 20
    seconds for the subjects who were drinking gin with the tonic.
Man: So, I’m really just repeating what’s on the chart, but I’m explaining it in words.
Woman: Right. But, here’s the thing—you shouldn’t draw any conclusions in the results section.
    Just the facts.
Man: You mean, just what happened in the experiment.
Woman: Right. You have to save the conclusions for the discussin sectin.  Q22
Man: Okay. So in the conclusions, I might say that driving after having four drinks... if you
    have four drinks, you probably won’t be in a position to avoid an accident.
Woman: That’s good. And you could also point out problems in the research, if there are any, or
    suggestions for future research.
Man: Like doing the experiment with beer instead of hard liquor or repeating the same
    experiment with three drinks instead of four.
Woman: Sure. That’s the idea.