I have a related question. I have undated data in two series x and y, with 50 observations in each. There is no overlap in their respective observation numbers i.e. x runs from 1-50 and y from 51-100. I would like to draw a random sample from each, say 20, and combine the two random samples into a series consisting of the 40 observations. I've played around with this but can't quite seem to get there. Any suggestions would be very welcome. Thanks.