New Study:
Supervised exercise versus non-supervised exercise for reducing weight in obese adults.
Nicolaï SP et al
J Sports Med Phys Fitness. 2009 Mar;49(1):85-90.
This research study looked at the effects of training with supervision/instruction (ie a qualified trainer) versus "advice".
Two groups - Group one met a trainer twice a week for 4 months (as a group).
Group two received basic advice to increase physical activity and access to a fully equipped gym (any exercise they did was unsupervised).
The group with supervision lost on average 362% more fat over a 4-month period. (Total fat loss of 13.4lbs in the coached group - and only 3.7lbs in the advice only group).
Interesting side note - They made these improvements without ANY dietary advice given.
This is not a new finding, nor is it that surprising as a good coach or training partner/team can really help with your results. The fact that the group exercised together only served to improve those results further (as social support is one of the key variables in fat loss success) so I'm not sure that the supervision and instruction alone was entirely responsible for the enhanced results - or the social support was part of it. However, it's clear that together it's an unbeatable combination.
You'd also have to assume that there was more instruction in the supervised group as there was more contact with instructors.
Another study came out a few years ago showing that a participant would exercise 30% harder or more (can't remember the exact number) with someone just watching them (i.e. not offering any instruction or encouragement). You can imagine how that could translate into much greater results, when added with a good program, great coaching, great social support and great instruction.
If you are serious about changing your body - hire a coach, or get a great training partner or team to help you.