Interval Measurement: It  is a  scale based on equal units of measurement indicating how much of  a given attribute is present  in the subject. It exhibits equal differences  ia amounts. Example  : Temperature limitation  of  this scale  is  : it  fails to measure absolute magnitude. The  scale does not  have a true zero. Continues variable yield interval measurement. Analytical possibilities  are greater. The data  can be meaningfully  added, subtracted and averaged.  i.e.  If  (a = b) and  (b = c), then (a -  c).  

The interval from  a  to  c =3-1  = 2. The interval from  c to d =4-3  = 1. We  can add this  (3-1)  + (4-3)  = 2+1 = 3. Now  note  that the interval from a  to d = 4-1  = 3. Expressed in an equation  (d-a)  = (c-a)  + (d-c).  

The statistics applicable are: 

-  can explore mathematical relationship,  such as Mean,  SD (Standard Deviation), product moment correlation, Standard Error, Variance  etc.