  Inspection of the periodograms of TESS delta Scuti stars indicates that there
is little, if any, similarity between the frequencies of stars in the same
region of the H-R diagram. This is difficult to understand because pulsation
models predict that stars with similar physical parameters should have similar
frequencies. To investigate the problem, a quantitative measure of similarity
between frequency patterns is described. When applied to non-adiabatic
pulsation models with similar temperatures and luminosities, a strong
correlation is found between the frequency patterns, as expected. The
correlation increases when rotational frequency splitting is included. When
applied to observations, no correlation can be found, confirming the impression
from visual inspection. It seems that each star has its own unique frequency
pattern, unrelated to its position in the H-R diagram. This presents a problem
in our understanding of stellar pulsation. The existence of a period-luminosity
law and the effect of combination frequencies in delta Scuti stars is briefly
discussed.
