There are two key things in software testing which are used to share the importance of a bug among the team and to fix it accordingly. They are:

1)     Defect Severity

2)     Defect Priority

Definition –

1)  Severity:

It is the extent to which the defect can affect the software. In other words, it defines the impact that a given defect has on the system. Higher effect on the system functionality will lead to the assignment of higher severity to the bug. Like, an error which occurs on the basic functionality of the application and will not allow the user to use the system will be a high severity bug.

2)  Priority:

Priority defines the order in which we should resolve a defect. Should we fix it now, or can it wait? If high priority is mentioned, then the developer has to fix it at the earliest. The priority status is set based on the customer requirements. In short, higher the number of users getting affected by the defect, higher will be the priority.

Types –

Priority can be of following types:

• Critical: The defect that must be resolved immediately because the defect is blocking a functionality or the product is affected severely. The system cannot be used until the repair has been done.
• High: The defect that must be resolved as soon as possible because the defect is affecting the application or the product severely.
• Medium: The defect which should be resolved in the normal course of development activities. It can wait until a new build or version is created.
• Low: The defect whose repair can be deferred until after more serious defects have been fixed.

Severity can be of following types:

• Critical: The defect that results in the termination of the complete system or one or more component of the system and blocks the user. The failed function is unusable and there is no acceptable alternative method to achieve the required results then the severity will be stated as critical.
• Major: The defect that results in the termination of the complete system or one or more component of the system and blocks the user. The failed function is unusable but there exists an acceptable alternative method to achieve the required results then the severity will be stated as major.
• Moderate: The defect that does not result in the termination, but causes the system to produce incorrect, incomplete or inconsistent results then the severity will be stated as moderate.
• Low: The defect that does not result in the termination and does not damage the usability of the system and is related to the enhancement of the look and feel then the severity is stated as low.

Difference –

Priority	Severity
Defect Priority is defined as the order in which the developer should resolve a defect	Defect Severity is defined as the degree of impact that a defect has on the operation of the product
·   Priority is categorized into three types Critical High Medium Low	Severity are categorized into four types Critical Major Moderate Minor
Priority is associated with scheduling	Severity is associated with functionality or standards
Priority indicates how soon the bug should be fixed	Severity indicates the seriousness of the defect on the product functionality
Priority of defects is decided in consultation with the manager/client	Usually, QA engineer determines the severity level of the defect
Its value is subjective and can change over a period of time depending on the change in the project situation	Its value is objective and less likely to change
High priority and low severity status indicates that the defect has to be fixed on immediate basis but does not affect the application	High severity and low priority status indicates that the defect has to be fixed but not on immediate basis
Priority status is based on the customer requirements	Severity status is based on the technical aspect of the product

Examples –

 

High Priority & High Severity:

1. A site maintaining the student details, on saving record if it, doesn’t allow to save the record then this is high priority and high severity bug.
2. On an E-commerce website, every customer gets error message on the booking form and cannot place orders, or the product page throws an error and does not show product details.

High Priority & Low Severity:

1. The spelling mistakes that happens on the cover page or heading or title of an application.
2. The logo or name of the company is not displayed on the website. It is important to fix the issue as soon as possible, although it may not cause a lot of damage.

High Severity & Low Priority:

1. On tap of a link in the Privacy Policy page, the app is crashing. This is a high severity defect but very few users will be affected by it because not many people visit the Privacy Policy page.
2. Customers who use very old browsers cannot continue with their purchase of a product. Because the number of customers with very old browsers is very low, it is not a high priority to fix the issue

Low Priority and Low Severity:

1. Any cosmetic or spelling issues which is within a paragraph or in the report (Not on cover page, heading, title).
2. A page of less importance like, Terms and Conditions, loads slowly.

However, it should be mentioned in this context that a same bug for 2 different applications can have different severity and priority levels depending of the target audience and the main usage of the application. For example, for an application targeting to teach English Grammar, any spelling or grammatical error will be a high severity and high priority defect. Bu the same defect for an E-commerce application will be a low priority and a low severity defect. Also, when a large number of users report an issue, it is always of high priority but can have different severity levels.