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In this course, author Barton Poulson takes a practical, visual, and non-mathematical approach to the basics of statistical concepts and data analysis in SPSS, the statistical package for business, government, research, and academic organization. From importing spreadsheets to creating regression models to exporting presentation graphics, this course covers all the basics, with an emphasis on clarity, interpretation, communicability, and application.
One of the first steps in any data analysis is to thoroughly investigate each of your variables one at a time, that is, to get univariate analyses. I've already described one procedure for getting univariate information with the frequencies procedure, and that work for both categorical and scale variables. Another important option for univariate statistics is the Descriptives command. This command and the Frequencies command do a lot of the same things but there are some important differences. The most significant is that frequencies can work with Categorical variables and Scale variables but descriptives works only with Scale variables.
In this movie, I will highlight the similarities as well as point out some of the unique advantages of the Descriptives command. For this example I will be using the same data set that I used in the last one. That's information about the stocks in the NASDAQ index, NASDAQ.sav. To get the descriptives, I go up to Analyze, to Descriptive Statistics, to Descriptives. From here, I select the variables that I want. You will notice it doesn't list all of the variables. It only lists the ones that are numeric.
The symbol and the name variables, as well as industry, are text variables and they are categorical and it simply doesn't list them here. So I am going to take the two that I used in the last example. That's LastSale-- so I am just going to click to move that over to the right--and MarketCap-- I am also going to move that over to the right. Then what I can do is I can get options where I select the statistics that I want. Now, by default, the descriptives gives me the mean, the standard deviation, the minimum and the maximum, and these are a good list.
I can also get Kurtosis and Skewness if I want. What's important though is I cannot get the quartiles. I can't get the 1st quartile, or 25th percentile score, I can't get the 3rd quartile, or 75th percentile score, and I can't get the median, and for a skewed distribution those are important statistics. So that is one reason to sometimes use the Frequencies command over the descriptives, is if you need the median and the quartiles. But I will just click Continue. Now, I have another option here. You will see at the bottom-left it says Save standardized values as variables.
This is one of the big perks of the descriptives command. If you want to take a variable that is in some metric like dollars or an arbitrary metric that may be a foreign currency you're not familiar with, sometimes you want to save things as standardized variables. That makes it so that the mean is 0 and the standard deviation is 1, and the individual cases get scores that indicate how many standard deviations above or below the mean they are. These are also called Z scores. I have seen people demonstrate how to do this manually by calculating everything.
That's very tedious. The descriptives gives you one stop way of doing this: you simply click the box and it will add standardized values for these things. And we can see how many standard deviations above or below the mean some of the companies are on these items for last sale and market capitalization. Now, there is also an option here for Bootstrap. I am not going to get into that one because the Bootstrap is an add-in feature that you pay extra for in SPSS. I am just going to deal with the ones that come standard. So now I can press OK and what I get is a small table. In the Frequencies command, the variables were listed as columns across the top and the statistics were listed as rows down the side, but Descriptives, it's flipped around.
But what I have here is the number of cases that I have information on. So for last sale I have 2817 companies with information on that. The minimum value is $00.1, the maximum value is $1,000 or $1,132, the mean is 18.7, and the standard deviation is 34.65 and then you have similar statistics for market capitalization. Now, an interesting trick is if we go back to the data set, and you see that we have two new columns here at the end, ZLastSale for the Z score or standardized value, where you can see that most of the scores are close to 0 or 1. We do have a major outlier at 9.99.
That's nearly 10 standard deviations above the mean. That's Apple computers, where their stock is costs about 10 times as much as most others. And then we have Z market capitalization. That's, again, a Z score, how many standard deviations above or below, and then Apple is again 32 standard deviations above the mean on this particular one. Hopefully, from all of this you can see that the Descriptives command is a really useful way of getting a variety of univariate statistics for your data. Like the Frequencies command, it can give the mean, the standard deviation, minimum, maximum, and other statistics.
It can give you the standardized scores, which the Frequencies command can't do. On the other hand, Frequencies can give the percentile statistics like the quartiles and the median. It can give the mode, it can give frequency tables and charts, and it can work with string variables and categorical variables. Now, for these reasons I generally prefer to use the Frequencies command, but either one will get you a very long way towards a sound understanding of your data and a solid foundation for further analysis.
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