How would results vary between observing a population directly as opposed to observing them indirectly?

Direct observations are made on subjects by recording the behaviours they exhibit in their natural setting. The researcher can either be physically present to record these observations using a camera or can hide to avoid any interference in the data gathering process. Example, watching or recording a bird chirp.

Indirect observation involves recording the traces left by the subject to understand the reason behind the behaviours exhibited by a subject. Example,    observing the distribution of bird droppings to figure out their movement.

Thus, a direct observation will help identify the behaviours in a population and an indirect observation will establish the reason behind the observed behaviours in the population.

of those who start drinking before they're 15 will develop alcohol dependence

x would be the most and the least would be y because there are more fruit   and seeds than flock z has insects