but one faculty member from Emory's School of Public Health has an introductory manual. Finally, there are a number of commercial packages such as SAS, SPSS and many others. Most of the major commercial and free packages have many statistical procedures in common. The main reason to use free packages is probably the cost. Menu driven packages Many of the packages have some kind of opening menu that is used to get or enter the data, manipulate the data, and select the statistical analysis. Then after starting the program, generally data can be obtained, either from previously saved data sets, or importing from some other format. From this menu, data files in various formats can be imported. For example, if the data is in CSV form (text with commas between values), the program recognizes the format and creates a data set from the CSV file. Finally, the program can be used to do some analysis. In this analysis menu, the variables of interest can be selected, along with other options. Then the analysis is run and results are obtained. Command driven packages R can be used both in a menu-driven way and as a programming language and as an interpreter. Getting data Most packages are able to import data from Excel or CSV (text with commas separating values). One consideration is whether there are missing data. Some packages, like PSPP and MicrOsiris, can automatically deal with the missing data. So for example, say one set of data look like this: In this data set, Sam is missing his age, and Sally is missing whether she was born in the USA. When some packages, like PSPP or MicrOsiris, read in or import the original data set, the packages will recognize that those values are missing, and do their calculations accordingly. MicrOsiris automatically assigns 1.5 or 1.6 billion to blanks as missing, and these values are excluded from analysis. Other packages need a 'placeholder', such as '-9' where there are missing data. Before the package is used to read the data, the data set has to be edited to put in a placeholder where there are missing data. So for example: If the data set actually includes '-9', then when the data is being read in the program will have to be told when the -9 means missing data. Limitations of packages Most of the packages have limitations of some sort. Several