MECE is a best practice method to lay down a robust structure for your case interview

Use MECE segmentations to group information in a logical manner

MECE is a way of segmenting information into sub-elements that are mutually exclusive and collectively exhaustive. In other words, elements should “exclude” each other, i.e. be distinct and should “exhaust” the relevant field, i.e. contain everything that belongs to it. MECE as a method should be used when you craft an issue tree for your case structure. Doing so will help you avoid dependencies between different branches of the tree and thus sub-problems can be properly isolated.

Mutually Exclusive: aims at reducing complexity by avoiding overlaps. You need to make sure that the possible solutions or groups are not accidentally considered twice. Proving exclusiveness forces you to carefully look at each option, consequently leading you to a much deeper understanding of the issue.

Collectively Exhaustive: aims at ensuring a comprehensive collection without leaving alternatives. Exhaustive means that all possible options have been considered. The trick is to divide a problem into categories with a finite number of general groups. For example, if you need to make “means of transportation” MECE, don't start just collecting “per foot, train, plane, bicyle, bike etc.” Try to find categories, because this practice will prevent you from forgetting important options: wheels vs. no wheels OR air vs. water vs. land.

Learn the difference between MECE and non-MECE

Another example for a MECE breakdown is dividing the population into subsets of age:

• A group of people below 50 years and another group of 50 years and above is MECE. No person can be in both groups - mutually exclusive - but every person is in either one of them – collectively exhaustive

• A group of people below 50 years and another group with people from 40 to 70 years is not MECE, as people from 40 to 50 years belong to both groups - they are not mutually exclusive – and people above 70 are in neither one of the groups – they are not collectively exhaustive.

Be MECE in profitability cases to split costs and revenue in an issue tree

The MECE methodology is famously used at McKinsey. McKinsey interviewers along with other firms will especially look whether you are able to structure information in a MECE format. During a case interview, MECE is also helpful for calculation. Many well-known frameworks, such as Cost-Benefit, 4Cs, Porter’s Five Forces are all designed with the MECE principle in mind.

The example below, from the "Startup vs. Consulting case" on PrepLounge, shows how costs are typically broken down in a MECE way. Subsequently, an issue tree-like format is used to break down the estimation into its basic components for easier approximation of data.