data is where we have columns representing variables and rows representing the cases or observations.
numerical in nature or quantitative in nature
 when we look at data, data is broadly classified into two
categories; categorical data and numerical data.  So, when we look at categorical data, these are also called as qualitative variables.
What do we mean by group membership?
 Again we go back to our student data, let us look at gender.
 Gender is a categorical variable.
 I have two categories here. I can classify any observation into one of these two categories.
 So, it is a group membership. Similarly, when I look at board I have a category which is a State Board, I have ICSE, I have CBSE. So, again you can see that this categorical variable has three categories and any observation can be categorized into one of these three groups.
 So we are giving membership of an observation to a particular group in that particular variable.
 So, this category has groups.
 Let us go to the hospital data.
 You see that blood group every patient is either an O positive or an O negative or a B positive or a A positive or A negative.
 So, you can see that there are many blood groups I again this is a categorical variable; gender is a categorical variable.
numerical data
 When we have numerical data, numerical data is also
called quantitative variables.  Here I can talk about numerical properties of data.
 we need to understand what is the scale that defines the numerical data.
 Again we have already emphasized on the point that when you have numerical data which take units.

within numerical data, I could have discrete data and I could have continuous data.
 We need to ensure that the variable is measured across all observations and shares a common unit.
Time Series Data and Cross sectional
 Apart from categorical data and numerical data, we also have data which where which are referred to as time series data.
 we refer to as a time series data where the data on a particular variable; this could be the quantity procured on potato is obtained the variable is the same.
 what we refer to as a time series data whereas, cross sectional data is the data which is observed at the same time.
Summary
 So, we will to broadly classify, we should know that given data we classify them broadly as categorical or numerical.
 So, whenever we are presented with a data set, we should be able to classify all the variables in the data set as a categorical variable or a numerical variable.
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