AbstractOn the basis of total ozone, Umkehr data and ozone soundings with Mast‐Brewer sondes at Aspendale, it is shown that the systematic effect of selectivity in the days on which Umkehr observations are made may be as large as 10 to 35 per cent of the annual mean content of layers 2 and 3. This selectivity effect varies with time, and is reasonably explained in terms of a correlation between ozone content in the lower stratosphere and cloudiness. Caution with regard to the use of Umkehr‐derived mean ozone distributions in climatological studies is therefore urged, and the need for further research is pointed out. The statistical limitations of mean distributions derived from ozone‐sonde flights are also discussed, with particular reference to quasi‐periodic inf