When calculating PSSUQ scores, what happens with NA answers?
PSSUQ (Post Study System Usability Questionnaire) is a post-task measurement tool which is used to assess system's usability. Current, third version has 16 statements. Each statement has a likert scale from 1 (strongly agree) to 7 (strongly disagree). Each statement also has NA (not applicable) option.
PSSUQ has an overall score which is calculated by averaging the scores given to statements.
User strongly agrees on every statement, PSSUQ overall score is therefore 1.
PSSUQ also has sub-scales for system usefulness (SysUse), information quality (InfoQual) and interface quality (IntQual). Their scores are calculated as:
- SysUse: averaging scores of statements 1 to 6
- InfoQual: averaging scores of statements 7 to 12
- IntQual: averaging scores of statements 13 to 15.
But how should I handle statements that user thinks are not applicable? Do they get the value 0 or do I just ignore them from the calculation? For example in IntQual which has only three statements if NA gets the value 0:
User strongly agrees on two statements and one statement does not apply, IntQual score is 0,667.
If NA is discarded:
User strongly agrees on two statements and one statement does not apply, IntQual score is 1.