The technology of crop production should provide high yields under varying environmental conditions typical for the geographical zone of production. However, crop yields may fluctuate from year to year. As long as these fluctuations are small, the technology reliably delivers yields close to the expectations, but occasional extreme low or high yields can cause serious concern for the farmers. Thus the fluctuations of yields should be kept within reasonable limits. The level of fluctuation is usually measured by statistical dispersion indicators, e.g. standard deviation, or coefficient of variation. Farmers may well tolerate small yield fluctuations as acceptable uncertainties of crop production, but a few extremely low or high yields may be disastrous for them. The present paper introduces an adjusted measure of a yield stability index and tests it for 10 countries and 18 crops. These crops include the main staple crops of the world, including cereals and vegetables. The countries selected are the main agricultural producers of the EU and a few large non-European countries. Results are compared for two time periods, 1961-2000 and 2004-2016, to show which crops are produced with the most reliable technologies in various countries. The adjusted Yield Stability Index was computed and compared for two time periods, 1961-2000 and 2004-2016. The results show that this index gives a meaningful measure of yield stability, which gives a more subtle indication of instability than the usual measures of dispersion. The positive values of the index can indicate a stable technology, i.e. one that can respond to the year-by-year variations of environmental conditions, and the suitability of the actual crop to the actual geographical environment, while the negative index shows that the crop may not be suitable, or the applied technology is not good enough. These considerations should be taken into account when deciding about cropping structures and R&D directions.