open interest vs contracts-relationship

#1
Sirs,
this is my first post and my goodwishes to all members.I may add that I am a newbie as regards derivatives.I shall be thankful if my query is answered.
As I know open interest is the number of contracts(buyor sell) pending as at the end of day.Now while perusing the data of Bank Nifty below I could not grasp the relationship between contracts traded on the day and the increase or decrease in open interest.

Code:
CONTRACTS		OPEN_INT	CHG_IN_OI 
1				50	0	19-Feb-09
1				100	50	20-Feb-09
6				150	50	24-Feb-09
0				150	0	25-Feb-09
For example: In the file "OIvsCONTRACTS.XLS" ATTACHED- Only 1 contract was traded on 19-feb-2009 and there was no change in open interest presumably due to offsetting of existing contract.However, on 20-feb-2009,again 1 contract has been traded and open interest has changed by 50 and on 24-2-2009, 6 contracts have been traded and change in open interest is still 50.how is it?Please help me to comprehend the mechanisim involved in this.And what is the relationship between OI and Ccntracts traded on the day.
2. Suppose I wish to download fobhavcopy for the one month and I have to daily download the file.Is there a software or application or method by which I can download data say for the month?
3. Is there a method to extract the required data from the Bhavcopy to excel i.e. bhavcopy contains the data for all the scrips but I want to see data for my scrips say 20 in all.?
thanks
vkwd7
 

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