Bbands using Ema instead of sma

[you]

Hi

Can I use ema instead of default sma for bbands indicator? I tried to pass bt.indicators.MovAv.EMA to param movav when creating bands , the code starts and run without isssue but the bbands top and bot values are much bigger than the expected value.. it works perfectly fine with sma.

Thanks

[backtrader]

As you have already used and documented under Docs - Indicators Reference the BollingerBands can use any moving average for the calculations (it could actually use any indicator which you throw at it)

The questions:

but the bbands top and bot values are much bigger than the expected value

• What is the expected value?
• Do you have a reference to compare against?

[you]

Hi

Here is one example:
Live data from IB on AUD-201706-GLOBEX

bbands with SMA: (period=20, devfactor=2.0)
top: 0.751921 mid: 0.751146 bot: 0.750371 2017-04-07T14:21:30

bbands with EMA: (period=20, devfactor=2.0, movav=bt.indicators.MovAv.EMA)
top: 0.792232 mid: 0.751423 bot: 0.710615 2017-04-07T14:21:30

Compare to IB, the EMA top and bot point in the program are much higher (IB value : top 0.7524 bot: 0.7504) but the mid point matches at 0.751423. Also SMA values matches with IB.

Any suggestion how I can further check on this?

[backtrader]

The StandardDeviation would use a SimpleMovingAverage in all cases. This seems to have a jumping-jack effect for the values.

This commit makes the BollingerBands pass the chosen moving average: https://github.com/mementum/backtrader/commit/817fed29ccb7374e38bd0ab700f96fd4737ba514

After the commit:

Before the commit

This will be the final straw for a release later today.

[you]

Thanks - will test with the latest version