EU-28 countries ranked by Hofstede's cultural dimensions

I do not know because I have not yet read his book(s), though I was able to find a total number of correspondents for the original study of IBM employees on a third party website:

In 1980, Geert Hofstede's book, Culture's Consequences: International Differences in Work-Related Values was published. In this book, Hofstede describes the culmination of a research project begun in the 1960's for the HERMES corporation, later revealed to be IBM (Hofstede and Bond, 1988). The original data set, derived from 117,000 questionnaires in 20 different languages comparing 88,000 respondents from 66 countries in 50 occupations, was eventually stabilized to include 53 countries and regions (Hofstede, 1983a).

The reference at the end of the paragraph is:

Hofstede, Geert (1983a), "Dimensions of National Cultures in Fifty Countries and Three Regions," in Expiscations in Cross-Cultural Psychology, J.B. Deregowski, S. Dziurawiec, and R.C. Annis, eds., Lisse, Netherlands: Swets & Zeitlinger, 335-355.

This is probably the publication that will be the most informative in this regard. Google Scholar gives 985 citations in academic texts for that book.

On Geert Hofstede's own website there is also this tidbit that I am interested in but where they again fail to mention the exact studies:

Between 1990 and 2002, these dimensions [The original four: Power distance, individualism/collectivism, masculinity/femininity, uncertainty avoidance] were largely replicated in six other cross-national studies on very different populations from consumers to airline pilots, covering between 14 and 28 countries.

And:

The country scores on the six dimensions are statistically correlated with a multitude of other data about the countries. For example, power distance is correlated with the use of violence in domestic politics and with income inequality in a country. Uncertainty avoidance is associated with Roman Catholicism and with the legal obligation in developed countries for citizens to carry identity cards. Individualism is correlated with national wealth and with mobility between social classes from one generation to the next. Masculinity is correlated negatively with the percent of women in democratically elected governments. Long-term orientation is correlated with school results in international comparisons. Indulgence is correlated with sexual freedom and a call for human rights like free expression of opinions.

So, I am of the same view that you that some critical facts are lacking to have sufficient insights into this. I think he might be holding it back from the public in order to push book sales. Maybe (again, haven't read them).

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