What is Proportionality?
Proportionality is a privacy principle that personal data collected and processed should be adequate, relevant, and limited to that necessary for purpose processed. It is one of the data processing principles of the GDPR.
Proportionality has multiple dimensions. Data collected and used should be adequate, because collecting too little information may lead to incorrect or incomplete information on a data subject. It should also be relevant and limited to what is necessary in relation to the purposes for which it is collected and processed (‘data minimization’), both in terms of scope and time (data retention).
The proportionality principles consideration of the amount of data to be collected. If excessive data is collected in relation to purposes then it is disproportionate. Examples: Using biometric data like fingerprints to identify individuals when identity cards would suffice. Proportionality as a principle applies in the area of employers monitoring employees. Data Privacy Impact Assessments should include an examination of proportionality of the processing operations in relation to purposes.
See GDPR, Article 5, Principles relating to processing of personal data