The McClellan oscillator and McClellan summation index were created in the late 1960s by Sherman and Marion McClellan. The oscillator is the product of subtracting one moving average from another.
These two averages are both based on the difference between the number of advancing shares and declining shares. One is a faster average covering a 19-day period and the other a slower one taking 39 days into account. Both are skewed to give greater weight to recent days' numbers.
Today's oscillator reading = 19-day exponential moving average - 39-day exponential moving average
By compiling a running total of oscillator readings, we get to the McClellan Summation Index. Originally, a reading of 1000 was considered to be neutral for this indicator, but a modernised version uses 0 for this purpose. You should always be careful to check which variant you are looking at.
Although you can calculate these indicators for yourself merely by using a spreadsheet, the process is fiddly and time-consuming. And there really is no need for you to do so. A decent charting software package will come with McClellan oscillators and summation indices built in for several indices, stretching back for at least a few years.
If you want to research McClellan charts going back as far as the 1920s, you can subscribe to the service provided by www.decisionpoint.com. The McClellans' own book on their indicators is available from their website www.mcoscillator.com, along with plenty of free information on the markets.
