Using standard deviation on two mean values of two different instruments

Hi all,

I'm making a research and doing a final data analysis. The research was undertaken using a questionnaire and then structured interviews. Questions were basically the same. In the questionnaire I surveyed a theme and, using a Likert scale, I got a mean result of 4 (using a scale from 0 to 10). The same topic was studied using interviews with quantitative analysis. Here the result is 7.8. The questionnaire had 193 samples, interview only 4. The interview were done with the aim to acquire data to confirm findings. As the difference between both values is too high, results cannot be confirmed.

Can I use the standard deviation (calculated in 2.68) to affirm that, accepting a maximum deviation of 1, this theme cannot be established and needs more analysis?

Generalizing, the statistical question is if the standard deviation of samples can be used on two mean values acquired with different instruments on a different population.