What is the meaning of "backfill"?

[excite]

Please forgive my ignorance, but can someone tell me the meaning of "backfill", tried google, but still not very clear. Thanks.

 

[Wysiwyg]

Please forgive my ignorance, but can someone tell me the meaning of "backfill", tried google, but still not very clear. Thanks.

In reference to data it means to "fill" the space between last time data was downloaded to the present time data is downloaded. So there isn't a void in between.

 

[excite]

Thanks Wysiwyg, still wondering why we need this specific term "backfill", as I understand that every time the data will be updated to the current, and naturally fill in the gap.
Looks like this "backfill" is quite important for intra-day trading?

 

[howardbandy]

Hi Excite --

As Wysiwyg describes, backfilling a data series makes the series complete by filling in any bars that were missing. If you are running a trading system that depends on formulas, and those formulas refer to previous data, it is important that all of the data be present.

Thanks,
Howard

 

[excite]

Thanks Howard,
So time can be saved by backfilling fewer records that occurred since last update up to current time, instead of reloading records for the entire day. Is that the purpose of backfill?

 

[howardbandy]

Hi Excite --

The purpose of backfilling is to complete the historical data series so that calculations of indicators have all of the data. The calculation will be made using whatever data exists -- the computer does not know whether there was trading on any any particular day or at any particular time. Imagine computing a moving average of 20 daily bars. If we have the price for the latest bar, and some prices for days in last month, but nothing in between, the calculation will be made but will be inaccurate. Backfilling completes the series so that the prices for the correct and most recent 20 days are used.

Thanks,
Howard