1997 Version of ESS
The original version of the ESS was first published in 1991. However, it soon became clear that some people did not answer all the questions, for whatever reason. They may not have had much experience in some of the situations described in ESS items, and they may not have been able to provide an accurate assessment of their dozing behaviour in those situations. However, if one question is not answered, the whole questionnaire is invalid. It is not possible to interpolate answers, and hence item-scores, for individual items. This meant that up to about 5 % of ESS scores were invalid in some series.
In 1997, an extra sentence of instructions was added to the ESS, as follows:
‘It is important that you answer each question as best you can’.
With this exhortation, nearly everyone was able to give an estimate of their dozing behaviour in all ESS situations. As a result, the frequency of invalid ESS scores because of missed item-responses was reduced to much less than 1%.
The 1997 version of the ESS is the standard one for all to use, in English and in translation. It is available in pdf here 
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Sample Only
