Short-term memory refers to memories which remain for around 20-30 seconds. It’s a very short time-frame and there’s limits to the amount of them as well. Some experiments performed by Petersen and Petersen (1959) suggest that the maximum capacity of Short-term memory is around 7(+/- 2) chunks of information (also called The Seven Plus or Minus Two Theory).
However, this is something really difficult to measure, due to the significance that an information may or may not have for a person. For example, four random numbers can be four pieces of information, but they could also be a date which might mean something for a specific person, so it will become just one piece of information for such an individual, and so, theoretically, that person would be able to remember more pieces of information that another one, for whom the numbers didn’t get memorized as a date, since it didn’t mean anything to them.
Based on this information, scientists determined chunking as a method to improve our Short-term Memory capacity. Giving some meaning to the information, as well as a favorable context of learning, a subject would be able to retain a somewhat larger amount of information.
In the same way, giving some tips to the subject, would improve his or her ability to recall the information.
Serial Position Effect
There are a lot of circumstances that can affect our Short-term memory capacity and the serial position is definitely one of them.
Some experiments revealed that when some words are presented in a row, or sequence, the subject tends to remember more efficiently the first ones (primacy effect) and last ones (recency effect), while the ones in the middle of the series are frequently forgotten.
This can have some practical implications, as, for example, it could be the explanation for the fact that the first impressions you have, regarding other people, can have such a great impact on your perception of them.