Update text descriptions and reference

src/dials/algorithms/symmetry/cosym/__init__.py

@@ -93,7 +93,7 @@
   .help = "If not None, a weights matrix is used in the cosym procedure."
           "weights=count uses the number of reflections used to calculate a pairwise correlation coefficient as its weight"
           "weights=standard_error uses the reciprocal of the standard error as the weight. The standard error is given by"
-          "(1-CC*2)/sqrt(n-2), where (n-2) are the degrees of freedom in a pairwise CC calculation."
+          "(1-CC*2)/sqrt(N), where N=(n-2) or N=(neff-1) depending on the cc_weights option."
 cc_weights = None sigma
   .type = choice
   .help = "If not None, a weighted cc-half formula is used for calculating pairwise correlation coefficients and degrees of"

src/dials/algorithms/symmetry/cosym/target.py

@@ -179,8 +179,9 @@ def __init__(
             is to use no weights. If "count" is set, then weights are equal to the
             number of pairs of reflections used in calculating each value of the
             rij matrix. If "standard_error" is used, then weights are defined as
-            :math:`w_{ij} = 1/s`, where :math:`s = \sqrt{(1-r_{ij}^2)/(n-2)}`.
-            See also http://www.sjsu.edu/faculty/gerstman/StatPrimer/correlation.pdf.
+            :math:`w_{ij} = 1/s`, where :math:`s = (1-r_{ij}^2)/sqrt(N)`.
+            Where N=(n-2) or N=(neff-1) depending on the cc_weights option.
+            See also  https://doi.org/10.1525/collabra.87615.
           min_pairs (int): Only calculate the correlation coefficient between two
             datasets if they have more than `min_pairs` of common reflections.
           lattice_group (cctbx.sgtbx.space_group): Optionally set the lattice