a.Scatter Diagram: Scatter diagram is a special type of dot chart. Under this methods the given data are plotted in a graph paper in the form of dot. For each pair of or x and y values we plot a dot( or point) and thus we obtain many dots equal in number of observation. If now these plotted dotes or points show some trends either upwards or downwards then the two variables ( x and y) are said to be correlated or otherwise not correlated. If again the trend of points is upwards moving from lower left hand corner to upper right hand corner then correlation is positive ( r=+1) r is coefficient or correlation. On the other hand if movement is reverse i, e, dots move from upper left hand corner to lower right hand corner then correlation is negative (r= -1). The idea will be clear from diagram given below:
Positive Correlation Negative Correlation
In fig (a) the values of the two variable move in the same direction the correlation is positive and r= 1.
In figure (b) we find negative correlation and r= 1 as the values move in reverse direction.
In figures (c) we dont get any trend lying and heance it shows the absence of correlation and r= 0.