  Refractive interstellar scintillation (RISS) is thought to be the cause
behind a variety of phenomena seen at radio wavelengths in pulsars and compact
radio sources. Though there is substantial observational data to support
several consequences of it, the quantitative predictions from theories have not
been thoroughly tested. In this paper, data from our long-term scintillation
study of 18 pulsars are used to test the predictions. The fluctuations of
decorrelation bandwidth ($\nu_d$), scintillation time scale ($\tau_d$) and flux
density (F) are examined for their cross-correlations and compared with the
predictions. The theory predicts a strong correlation between $\nu_d$ and
$\tau_d$, and strong anti-correlations between $\nu_d$ and F, and $\tau_d$ and
F. For 5 pulsars, we see a reasonable agreement. There is considerable
difficulty in reconciling the results for the rest of the pulsars. Our analysis
shows the underlying noise sources can sometimes reduce the correlation, but
cannot cause an absence of correlation. It is also unlikely that the poor flux
correlations arise from a hitherto unrecognized intrinsic flux variations. For
PSR B0834+06, which shows anomalous behaviour of persistent drift slopes,
positive correlation is found between $\tau_d$ and the drift-corrected $\nu_d$.
Many pulsars show an anti-correlation between $\nu_d$ and the drift slope, and
this is in accordance with the simple models of RISS. The detections of
correlated variations of observables and a reasonable agreement between the
predicted and measured correlations for some pulsars confirm RISS as the
primary cause of the observed fluctuations. However, the complexity seen with
the detailed results suggests the necessity of more comprehensive theoretical
treatments for describing refractive fluctuations and their correlations.
