Types of Variable:

Very simply, a VARIABLE is a measurable characteristic that varies. It may change from group to group, person to person, or even within one person over time. There are six common variable types:
1)  INDEPENDENT VARIABLES
2)  DEPENDENT VARIABLES
3)   CONTROL VARIABLES
4)  MODERATOR VARIABLES
5)  INTERVENING VARIABLES
6)   EXTRANEOUS VARIABLE

1)      INDEPENDENT VARIABLES
Independent variables are variables which are manipulated or controlled or changed. Independent variable may be called factor and its variation is called levels.
In the example “a study of the effect of teacher praise on the reading achievement of second-graders”, the effect of praise, the researcher is trying to determine whether there is a cause-and-effect relationship, so the kind of praise is varied to see whether it produces different scores on the reading achievement test.  We call this a manipulated independent variable (treatment variable).  The amount and kind of praise is manipulated by the researcher.  The researcher could analyze the scores for boys and girls separately to see whether the results are the same for both genders.  In this case gender is a classifying or attributes independent variable. The researcher cannot manipulate gender, but can classify the children according to gender
2)      DEPENDENT VARIABLES
Dependent variables are the outcome variables and are the variables for which we calculate statistics. The variable which changes on account of independent variable is known as dependent variable.
Let us take the example, a study of the effect of teacher praise on the reading achievement of second-graders; the dependent variable is reading achievement.  We might compare the average reading achievement scores of second-graders in different praise conditions such as no praise, oral praise, written praise, and combined oral and written praise.
3)   CONTROL VARIABLES
Controlled variables are that is sometimes overlooked  by researchers, but it usually far more important than the independent or independent variables. These are unchanged variables.
For Example: Language learning and teaching are very complex processes. It is not possible to consider every variable in a single study. Therefore, the variables that are not measured in a particular study must be held constant, neutralized/balanced, or eliminated, so they will not have a biasing effect on the other variables. Variables that have been controlled in this way are called control variables.
4)  MODERATOR VARIABLES
A moderator variable changes the strength of an effect or relationship between two variables.  A moderator may increase the strength of a relationship, decrease the strength of a relationship, or change the direction of a relationship.
That factor which is measured, manipulated, or selected by the experimenter to discover whether it modifies the relationship of the independent variable to an observed phenomenon. It is a special type of independent variable.
The independent variable's relationship with the dependent variable may change under different conditions. That condition is the moderator variable. In a study of two methods of teaching reading, one of the methods of teaching reading may work better with boys than girls. Method of teaching reading is the independent variable and reading achievement is the dependent variable. Gender is the moderator variable because it moderates or changes the relationship between the independent variable (teaching method) and the dependent variable (reading achievement).
5)      INTERVENING VARIABLES
It is a variable whose existence is inferred but it cannot be measured. This is also known as mediator variables.
For example, if the use of a particular teaching technique is the independent variable and mastery of the objectives is the dependent variable, then the language learning processes used by the subjects are the intervening variables.
6)   EXTRANEOUS VARIABLE
Extraneous variables are undesirable variables that influence the relationship between the variables an researcher is examining.
Extraneous variables are independent variables that have not been controlled. They may or may not influence the results. One way to control an extraneous variable which might influence the results is to make it a constant (keep everyone in the study alike on that characteristic).