An effective resume provides insight to relevant experiences and accomplishments. Therefore, you do not need to include information on every employer you have worked with in the past. Determining what to include and what to leave out requires asking yourself the questions: "Is this relevant to the hiring employer and is there enough experience gained to showcase me in a positive light?" Other things to factor include how long ago you were employed by the employer, the length of time you were employed and your reason for departure.
Relevant Experience
When your previous employment is not applicable to the position you are seeking for now, adding it to your resume will dilute information and express to the employer how you are an irrelevant candidate for the job. You want to showcase how you have the knowledge and skills it is looking for. For instance, if you started your career in accounting and changed your profession to clinical science work, which is the type of job you are seeking now, an employer will not care much for your previous employment at an accounting firm or any side part-time jobs that are irrelevant to its field, according to a "U.S. News & World Report" article on common mistakes made on resumes.
Dates of Employment
When you have a long work history and have established yourself in a profession, employers will want to know more about what you have done in the recent years. According to "U.S. News & World Report," hiring employers are more interested in what you have done in the last eight to 12 years [Reference 2], and anything beyond that will bear much less weight. For instance, if you have been in the marketing profession for 10 years, it is not necessary to include information about the marketing internship you held after graduating from college. Your level of experience is far beyond the point of an intern, so what you did then will not matter much and the years of experience you have in the field is enough to substantiate what you have to offer to the employer.
Length of Employment
Having a brief period of employment where you do not have much to show for it in terms accomplishments or experience can be left off the resume. This is especially the case when leaving it out doesn't create a dramatic gap between your employment periods. Brief employment on a resume creates red flags for hiring employers. For instance, if you show a common theme of short term employment of only a couple of months here and there, a hiring employer may question if you are a dedicated worker and someone who can commit long term. According to TheLadders, as a general rule, if employment "lasted less than six months, you can leave it off your resume."
Reason for Departure
There are a number of reasons why people leave their job, but when your reason of departure is because you were fired, you have to take special care in determining whether or not to include it to your resume. It can reflect poorly on you. According to career advice from "U.S. News & World Report," leaving it off can help you avoid an unnecessary conversation that may cause concern to the hiring employer. Keep everything on your resume positive. While you are not stating on your resume that you were fired, including the employer you were fired by may open a can of worms if you are asked why you left that employer.
