The Pareto principle states that 20% of the input creates 80% of the result. I would not get too hung up on the numbers as absolute, the key here is the concept. Everything is not weighted equally. You do not do a unit of work and get an equal result from that unit of work. Some types of training or methods can and will contribute more than others. The challenge for us in coaching is to carefully evaluate what we are doing in our training design and implementation to determine where we are getting the most return. We may have a favorite exercise or a workout that we think works, but if you step back you may see that it is not doing what you think it is doing, it is not where the results are coming from. In fact the results may be coming from something or somewhere else. It would be nice if every training session or method contributed equally to the adaptive process but that just does not reflect reality.
According to the Pareto principle the majority of the results come from a small percentage of the work. So find out where the results are coming from and focus your efforts there for the best return. To use a cliché, separate the wheat from the chaff, focus on the need to do and minimize the nice to do. A corollary to this is that more is not that much better; optimize the volume of work to minimize the law of diminishing returns. Make your decisions in training based on the return, know where you best results are coming from and focus your efforts there.