Data points and datasets
In data analysis, it is convenient to think of the data as points of information. For example, in a collection of biographical data, each data point would contain information about one person. Consider the following data point:
("Adams", "John", "M", 26, 704601929)
It could represent a 26-year-old male named John Adams with ID number 704601929.
We call the individual data values in a data point fields (or attributes). Each of these values has its own type. The preceding example has five fields: three text and two numeric.
The sequence of data types for the fields of a data point is called its type signature. The type signature for the preceding example is (text, text, text, numeric, numeric). In Java, that type signature would be (String, String, String, int, int).
A dataset is a set of data points, all of which have the same type signature. For example, we could have a dataset that represents a group of people, each point representing a unique member of the group. Since all points of the set have the same type signature, we say that signature characterizes the dataset itself.
Null values
There is one special data value whose type is unspecified, and therefore may take the role of any type. That is the null value. It usually means unknown. So, for example, the preceding dataset described could contain the data point ("White", null, "F", 39, 440163867), which would represent a 39-year-old female with last name White and ID number 440163867 but whose first name is unknown (or unspecified).