orrelation is used to obtain data which are unique for each
run of the script and which are generated by nested queries.
Correlation provides the value to avoid errors arising out
of duplicate values and also optimizing the code (to avoid
nested queries). Automatic correlation is where we set some
rules for correlation. It can be application server
specific. Here values are replaced by data which are created
by these rules. In manual correlation, the value we want to
correlate is scanned and create correlation is used to
correlate.

We have to scan manually for the manual 
correlation...Another thing is that record 2 script and 
compare both by using WINDIFF tool.we can find from this 
which part of the script needs a correlation

Correlation is where the script is modified so that some of 
the hard coded values in the script are no longer hard 
coded.  Rather than have LoadRunner send the original value 
to the server, we may need to send different values. 

For example, the original recorded script may have included 
the server sending the client a session identification 
number.  Something to identify the client during that 
particular session.  This Session ID was hard coded into 
the script during recording. 

During replay, the server will send LoadRunner a new 
Session ID.  You need to capture this value, and 
incorporate it into the script so you can send it back to 
the server to correctly identify yourself for this new 
session.  If you leave the script unmodified, you will send 
the old hard coded Session ID to the server.  The server 
will look at it and think it invalid, or unknown, and so 
will not send the pages that have been requested.  
LoadRunner will not have successfully fooled the server 
into believing it is a client. 

Correlation is the capturing of dynamic values passed from 
the server to the client and back.  You save this captured 
value into a LoadRunner parameter, and then use this 
parameter in the script in place of the original value.  
During replay, LoadRunner will now listen to what the 
server sends to it, and when it makes requests of the 
server, send this new, valid value back to the server; thus 
fooling the server into believing it is talking to a real 
client.

the correlation describes the direction and strenght of a 
straight line relationship between two quantitative 
variables n usually writthen as "r"

correlation is the relationship between two variables

Correlation is where the script is modified so that some of
the hard-coded values in the script are no longer
hard-coded.  Rather then sending the original value to the
server, we may need to send different values. 
For example, the original recorded script may have included
the server sending the client a session identification
number, something to identify the client during that
particular session.  This Session ID was hard-coded into the
script during recording. 
During replay, the server will send the client a new Session
ID.  You need to capture this value, and incorporate it into
the script so you can send it back to the server to
correctly identify yourself for this new session.  If you
leave the script unmodified, you will send the old
hard-coded Session ID to the server.  The server will look
at it and think it invalid or unknown, and so will not send
the pages that have been requested.  Replaying a script with
an old Session ID will not successfully fool the server into
believing it is a client.
Correlation is the capturing of dynamic values passed from
the server to the client and back.  You save this captured
value into a parameter, and then use this parameter in the
script in place of the original value.  During replay, the
replay engine will now listen to what the server sends to
it, and when it makes requests of the server, send this new,
valid value back to the server; thus fooling the server into
believing it is talking to a real client.
Steps to Perform Manual Correlation

1.Record two copies of the script with the same business
process and data:
2.Record a script on the desired business process and save it.
3.Record another copy of the script, repeating all the steps
as in step 1a. As much as possible, during recording, enter
the same values in both scripts, for example, user ID,
password, and fields and edit selections.  
4.Identify the values to correlate with the help of the
WinDiff tool:
5.From the second script, go to Tools  Compare with Vuser,
and choose the first recorded script. 
6.WinDiff will open and display the two scripts side by
side.  It will automatically highlight the lines with
differences in them, showing the differences in red. (If
not, go to Options  View  Show inline differences.)
Tips: Error: "File no longer available." launching WinDiff		
7.Locate the first difference, take note of it, and search
the script open in VuGen for that difference.  That is the
original value hard-coded into the script that was different
in the second script.  Highlight it and copy it.