Can we use a scale even if one of the answer levels was not added? I have a set of 20 participants for a study who were given a 7-point Likert scale that was made incorrectly by a graduate student. The student accidentally did not add the 6th answer level. The scale for these 20 participants arguably has no validity now. However, I am hoping there is a way to use it for a pilot or preliminary poster. The study will continue for a couple years and is proposed to have 300 participants. But I would like to use these 20 participants for a small pilot or preliminary poster. Is there a way for me to still use that scale (missing the 6th answer option) and the 20 participants? 

I don't know if I can impute it because the data is not MCAR, the answer level was not added to the scale as it should have been. But can I weight the low and high-ends of it or something still? I would like to see if the measured construct from that scale relates to either another variable (such as Engagement) or the outcome variable. It is a longitudinal study, they come in once a week for three weeks. Each time they come in to the lab and use the online intervention they take the scale three times (i.e., Baseline, after first half of website, and then after second half of website), which means we have each participants using the scale 9 times.
I think you'll come unstuck if you don't have a balanced scale.

If you consider a typical five-point scale:


Strongly disagree
Disagree
Neither agree nor disagree
Agree
Strongly agree


If we forget to put Strongly disagree on the scale, it is positively biased, because it is heavier on the Agree side of the scale. If we forget to add Strongly agree we have a negatively biased scale.

If you intend the questionnaire to be used for research purposes, you'll need to ensure the scale is correct - otherwise it may affect your findings, and it will definitely cause people to question whether the outcome would have been the same as if the correct scale was used.