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Q: Minitab - calculating standard deviation of grouped data ( No Answer,   0 Comments )
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 Subject: Minitab - calculating standard deviation of grouped data Category: Science > Math Asked by: _len_-ga List Price: \$10.00 Posted: 16 Jul 2003 06:15 PDT Expires: 15 Aug 2003 06:15 PDT Question ID: 231578
 `How do I calculate a standard deviation of grouped data in Minitab?` Request for Question Clarification by elmarto-ga on 18 Jul 2003 10:34 PDT ```Hello len, Just to make sure I'll give you the correct answer, does your spreadsheet look something like this? Year Sales 2000 100 2000 150 2000 130 2001 210 2001 250 2002 200 so the "groups" here would be 2001 and 2002. What you want to do is calculate the standard deviation of elements in group 1 and the standard deviation of elements in group 2? Best regards, elmarto``` Clarification of Question by _len_-ga on 18 Jul 2003 12:01 PDT ```Thanks for the question. The grouping is based on a attribute rather than a date. I'm not sure it makes any difference, but it'd probably better if I explain it. Suppose I'm manufacturing widgets every day. The time it takes to make widgets is variable, though, for a variety of reasons we don't care about. Suppose I measure the amount of time it takes, on average, to make widgets each day, and it looks like this: Widgets Seconds per Day Made Widget ----- ------- ----------- 1 100 60 2 92 71 3 115 56 4 85 63 5 72 57 6 60 55 7 55 50 I don't know how long it takes, for example, to make any individual widget on Day 1. All I know is that it took 6000 seconds (100 * 60) to make 100 of them. I believe this is what makes the widgets "grouped." Using the number of widgets made on each day, plus the average time per day, I can calculate the weighted average for making widgets for the entire week. This is easy in Excel. I can also calculate in Excel the standard deviation if I know the formula for calculating the std dev of grouped data. My question is how I'd calculate the std dev of this grouped data in Minitab. Hope this helps. Thanks very much. Len``` Request for Question Clarification by elmarto-ga on 18 Jul 2003 19:37 PDT ```Hi again, len! OK, let me see if I understand. You're defining the widgets made in day 1 as group 1, the widgets made in day 2 as group 2, etc, right? If this is the case, then you can't calculate the std dev of each group with the given data. Let's take group 1. You only know that the average time is 60 seconds. But since you don't know how much time each of the widgets took, you can't calculate the std dev for this group. Same for the other groups. If you wanted to calculate the weighted std dev across groups, I think you would have to manipulate the columns in Excel, paste them in Minitab and calculate the basic std dev. That is, you would have to create a column with a hundred 60's, ninety-two 71's, etc. I don't see any other way to this (some other researcher may, though). Please do tell me if I didn't understand correctly what you meant by groups. Best wishes! elmarto``` Clarification of Question by _len_-ga on 19 Jul 2003 04:14 PDT ```Hi elmarto, Yes, I'd thought of doing a cut-and-paste from Excel to Minitab with one line per widget. The only problem is that I've got more than a million widgets to measure, and Excel can only handle a fraction of that. I may end up sticking with my hand-made calculations in Excel. I can show how the calculations are derived, and I think that's more important than seeing it done in a particular tool. Len``` Request for Question Clarification by elmarto-ga on 19 Jul 2003 09:28 PDT ```Hi len! Well, here's my last attempt. Have you tried the "Make Patterned Data" feature in the "Calc" menu? If you do't have many observations (i.e., many days in the example), this might work to make one observation per widget. So if you have one hundred 60's and seventy 80's, choose in the "Make Patterned Data" dialog box: Store patterned data in: some new column From first value: 60 To last value: 60 In steps of: 1 List each value: 100 times and the same for each day, always in a new column. Then you can use the "Stack Columns" option in the "Manip" menu to make one long column with all the new columns, and calculate the std dev of that column. Please tell me if this worked you. Best wishes! elmarto```