The combined SD value for 2 groups using formula in the Cochrane handbook and using online calculators ( https://www.statstodo.com/CombineMeansSDs_Pgm.php ) was the same.
However, when calculated using SPSS by using the 'compute mean' option first to combine 2 means and then using 'descriptives' option to get SD value, it was different from the one obtained using the previous method.
(Mean value remained same using all methods)
Which value should be reported?
The answer is in the Cochrane Handbook:
"6.5.2.10 Combining groups
Sometimes it is desirable to combine two reported subgroups into a single group. For example, a study may report results separately for men and women in each of the intervention groups. The formulae in Table 6.5.a can be used to combine numbers into a single sample size, mean and SD for each intervention group (i.e. combining across men and women in each intervention group in this example). Note that the rather complex-looking formula for the SD produces the SD of outcome measurements as if the combined group had never been divided into two. This SD is different from the usual pooled SD that is used to compute a confidence interval for a MD or as the denominator in computing the SMD. This usual pooled SD provides a within-subgroup SD rather than an SD for the combined group, so provides an underestimate of the desired SD."
Answer differs depending on the number of values used after decimals. One technique takes restricted number while the other takes as many as are allowed which makes difference in the final answer.
Thank you for responding, Prof. Radha kumari Challa . I would like to point out that the two values differed significantly and not just the values after the decimals. As Prof. Roberto Molteni mentioned, it was more likely due to the different formula that was used.
Thank you all, rounding off fractions cannot make much difference in the two values. However, this difference can occur, often due to the use of different formulas in calculating the values.
Thank you Dr. Zeinelabedeen Elsayed Alwafaai . This is what I inferred as well.
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