23. Explain systematic and stratified sampling. 24. Distinguish between census and sampling. 25. Calculate AM from the following data. Class: 0-10 10-20 20-30 30-40 40-50 Frequency: 3 12 20 10 ary 26. The first four moments of a distribution about the value 5 of the variables are 2, 20, 40 and 50. Find the mean, variance, third and fourth central moments. 27. Explain Sheppard's correction for moments. lege 28. Calculate median for the data given below. Class: Frequency: 8 0-6 7-13 14-20 21-27 28-34 35-41 1728 15 9 3