From two normal populations assumed to have the same variance, independent random samples of sizes 15 and 19 were drawn. The first sample (with n1 = 15) yielded mean and standard deviation 111.6 and 9.5, respectively, while the second sample (n2 = 19) gave mean and standard deviation 100.9 and 11.5, respectively. The estimated standard error of the difference in sample means is______.