The Runner's Track

Especially in the fields of nutrition and exercise there are many conflicting opinions and little concrete evidence. Body by Science for example cites a number of studies which show that in strength training one set per exercise can be just as effective as multiple sets - but there are other studies which show the opposite. You then have to look at those studies in greater detail - who were the participants, which exercises were used, what level of intensity, how much rest in between workouts, duration of the study etc.. 

Drew Baye summarized the proper approach well: Test it yourself - do your workouts, keep accurate records of your progress, and if you're not stronger with each workout, make some changes. In a nutshell, your workouts need to be as intense as possible - one set to muscular failure per exercise, and only a handful of exercises per workout. If you really increase the intensity like that, you'll only be able to handle about 20 minutes of exercise per workout, and it will take your body at least a couple of days to produce the proper adaptation. So while intensity always needs to be kept as high as possible, you have volume and frequency to tweak. No progress? Try less volume, and/or more recovery days.