  We analysed the combined Greenwich (1874-1976) and Solar Optical
Observatories Network (1977-2011) data on sunspot groups. The daily rate of
change of the area of a spot group is computed using the differences between
the epochs of the spot group observation on any two consecutive days during its
life-time and between the corrected whole spot areas of the spot group at these
epochs. Positive/negative value of the daily rate of change of the area of a
spot group represents the growth/decay rate of the spot group. We found that
the total amounts of growth and decay of spot groups whose life times > or = 2
days in a given time interval (say one-year) well correlate to the amount of
activity in the same interval. We have also found that there exists a
reasonably good correlation and an approximate linear relationship between the
logarithmic values of the decay rate and area of the spot group at the first
day of the corresponding consecutive days, largely suggesting that a
large/small area (magnetic flux) decreases in a faster/slower rate. There
exists a long-term variation (about 90-year) in the slope of the linear
relationship. The solar cycle variation in the decay of spot groups may have a
strong relationship with the corresponding variations in solar energetic
phenomena such as solar flare activity. The decay of spot groups may also
substantially contribute to the coherence relationship between the total solar
irradiance and the solar activity variations.
