It is quite common for students to get abbreviations wrong. This is particularly true when they are confused between use of the upper case and use of the lower case. There are abbreviations where both need to be used and the students get trapped in deciding which letters should be part of the upper case and which ones should be part of the lower case.
Every style guide lays down clear rules with regard to how abbreviations should be used. It is recommended that students get a copy of the latest edition of whichever style guide they are following and make sure that they follow its recommendations.
One of the style guides that is popular the world over are the rules laid down by the Modern Language Association. Its simplicity is one of the reasons that so many students use it. Its rules are quite easy to obtain either on the Internet or in book form.
It has laid down very specific rules with regard to how lower letter abbreviations should be used. The guide says that a period symbol should be used when the abbreviation ends in a lower letter. However, this rule has its exceptions in the form of Internet suffixes. When an Internet suffix is being used, the period sign should come at the start of the abbreviation.
Another exception needs to be made when degree names are being given. Degree names are a combination of upper case abbreviations and lower case abbreviations. PhD is a very good example or PsyD. It is important to note that there are no period signs after the use of this abbreviation.
There are also examples where the period signs are used in between letters especially when the time is given. There are no period signs when a symbol of measurement is being given. For instance, mph or miles per hour has no period sign at the end.