  An evaluation method supported by robust statistical analysis was used to
analyze historical measurements of $^{39}$Ar half-life. The method, which
combines the most frequent value (MFV) approach with bootstrap analysis,
provides a more reliable way to estimate the half-life of $^{39}$Ar. The
results show that the half-life is T1/2(MFV) = 268.2 + (3.1) - (2.9) years,
with an uncertainty corresponding to the 68% confidence level. This uncertainty
is three times smaller than the most precise re-calculated measurements by
Stoenner et al. (1965) and 2.7 times smaller than the adopted half-life value
in nuclear data sheets. Recently, the specific activity of the beta decay of
$^{39}$Ar in atmospheric argon was measured in various underground facilities.
Applying the MFV method to these measurements gives a specific activity of
SA($^{39}$Ar/Ar)(MFV) = 0.966 + (0.010) - (0.018) Bq/kg(atmAr), with an
uncertainty corresponding to the 68% confidence level. This paper also
discusses the method used to determine the half-life of $^{39}$Ar using the
specific activity of $^{39}$Ar in atmospheric argon.
