Screening requirement

Please note that if control A.7.1.1. (screening) from ISO 27001 is applicable to your ISMS, then you have to apply it to all candidates for employment (by sampling candidates you will not be fulfilling the requirements for the control).

What you can do to minimize effort is to apply screening only to roles that are related to unacceptable risks, or because legal requirements require the screening to be performed. Additionally, you can perform the screening only at late stages of the hiring process, when you will have fewer candidates to evaluate 

This article will provide you a further explanation:
- How to perform background checks according to ISO 27001 https://advisera.com/27001academy/blog/2018/03/26/how-to-perform-background-checks-according-to-iso-27001/