Imagine that you have one of the following problems:
lacking time management skills, suffering from stress, lacking interpersonal communication skills.
write a letter to ms brown for advice on a suitable course and get further information about: course duration, lecturer/trainer, starting date, certificate, etc.

answer; racism;

it might look like that the ideal way to research the health effects of trans fats is through a controlled experimental feeding trial. but these kinds of trial have several important limitations that the researcher must look into:

short in duration (this factor usually checks up on the intermediary risk factors rather than the actual outcomes of the disease)

limited dietary changes

only makes use of healthy participants