# PDM fails at math?

Hmm. I can do better than this in my head (and I failed math in high school).

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Wow. Can you adjust it manually?

Sorry my friendâ€¦ but youâ€™re still failing;) jk unless I donâ€™t understand what youâ€™re sayingâ€¦ but 5.5 plus 5.55 does equal 11.05

I think the error is the 40/60 splitâ€¦ 40% of 11 is 4.4, 60% is 6.6

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The extended bolus display is better as units than it is for percentage. The delivery will give you exactly what it shows, 5.50 and 5.55 units. But if itâ€™s set for units, the display will always make more sense.

So you will get 5.50 and 5.55. But just switch the display to units and you will be better off.

BTW, the reason yours is having a problem with the math is because itâ€™s a Canadian PDM.

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So it does. Thank you, @Eric. I guess I have to do a separate calculation either way and then tell the PDM either units or percentage (though it would be nice if it would accurately figure it out for me), but why would the two settings give different numbers? Makes me worry what else the PDM doesnâ€™t calculate accurately â€¦

I guess they give you a choice because people do the calcs differently.

There are better ways do do things for everyone. Like for temp basal, % makes more sense for me. The reason is - if you use units, it will stay with that number throughout the length of the temp basal. If your normal basal changes amounts, do you want your temp basal to stay with the normal fluctuations or stay flat? For me, I want my temp basal to follow the same up and down, but just at a higher or lower amount.

So itâ€™s just a matter of trying all the options.

Yes, there are a few quirks Iâ€™ve noticed with math. One of them is with temp basal. They round things weirdly because of the 0.05 unit increment. But rest assured, it always delivers what it should. Iâ€™ve counted clicks through every possible scenario. Iâ€™m comfortable that regardless of any display anomaly, itâ€™s delivering correctly.

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As far as the total dose goes, yes. But as far as the dose delivered in the first segment, no, regardless whether itâ€™s set for units or percentage. I donâ€™t particularly care whether itâ€™s 30% or 35% or 40% or whatever (unless Iâ€™m following a formula that I know works for something like pizza). For several reasons,* I donâ€™t want to deliver more than 5U up front, so to be on the safe side I aim for 4.5U when choosing my percentage. But using the units setting this morning, when I said deliver 4.50 units first and the rest over an hour, the â€śDelivering bolusâ€ť screen said it was giving me 5.10 units up front. A bit of a nuisance.

I only spotted this problem a few nights ago, when a 30% up-front dose was significantly higher than the 70% extended, as though the PDM had got the two reversed. Wish Iâ€™d had the camera then.

*To prevent tunneling, and also, since Iâ€™ve been doing this, I havenâ€™t had any â€śthird day problemâ€ť highs.

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@Beacher have you tried calling Omnipod about this? I use extended boluses often and Iâ€™ve never noticed this. I just looked through my history and all the calcs seem to be correct.

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I am still curious about this. I would like to try and repro this and see if I can get the same kind of wacky numbers.

I can setup my PDM to use % for extended bolus instead of units, and also change my numbers to be like yours and do the same thing and see if it looks like yours.

Do you have carb number, BG number, and IC number you want to give me to try?

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Thanks, Eric. I think this is the first one that I noticed was reversed (Iâ€™ve re-entered a whole bunch of meals, but this one looks familiar):
BG 9.8, carb 47, IC 1:7
Suggested bolus was 8.95, to which I added 1U. I wanted 4.5 up front, so 4.5/9.95 is 45%. I entered that and the PDM said 45% = 5.75 and 55% = 4.20. Changing the duration of the extended portion doesnâ€™t change the numbers.

The numbers that resulted in what I took the above photo of: BG 9.3, carb 65, IC 1:7
Suggested bolus 11.05 (11.35 minus 0.30 IOB).
When I put those numbers in today, I get 40% = 5.65 and 60% is 5.40. The only thing different is the IOB, which would change the total dose, not the percent breakdown.

Hereâ€™s one more, from this morning: BG 9.8, carb 50, IC 1:5
Suggested bolus 12.35, to which I add 0.5
4.5U is 35%, but the PDM ends up saying 35% = 6.00 and 65% is 6.85

Anyway, Iâ€™ll be interested to see what your PDM says. And just to note, Iâ€™m not normally in the 9â€™s before a meal. Maybe this is the PDMâ€™s punishment for being high.

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What about target BG? I just put in something bogus to try this, not sure if that makes a differenceâ€¦

And also correction factor? Maybe that messes with it some?

Target 5.5. CorrF 1.9 in the first example, 1.8 in the second and third.

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@Beacher

Correction bolus canâ€™t be extended. Only meal bolus can be extended.

So it matters on the BG number you enter because that part (the correction) will not be extended, so the percentage doesnâ€™t include that.

Here are some examples I did with different BG numbers just to demonstrate the concept. I kept carbs the same.

A simple example. Suppose you have a BG that requires a 1.0 unit correction. And you are eating carbs that need 1.0 units. That dose total would be 2.0 units.

If you do 50% now and 50% extended, you might think 50% of 2.0 units. But thatâ€™s not right. Since 1.0 units is correction, that is not included.
So it would be:
2.0 (total) - 1.0 (correction) = 1.0 which is the only part that is extended at 50%.

Make sense?

EDIT
I changed it back to units and this screen makes the math more obvious:

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Interesting. We never use a BG correction calculated by the PDM, so this may be why we have never seen this behavior.

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Definitely makes sense! And it explains why Iâ€™ve only noticed the disparities when Iâ€™m running higher, not when Iâ€™m starting out near target â€“ because then thereâ€™s no (or not significant) correction involved.

Even in the units setting, you have to account for the correction, because if you say deliver 4.50U, the final screen might have changed that to 5.40, more than I want to deliver up front. It might be easier to stick with the percentage setting and just keep going back to lower the percentage until Iâ€™m near my 4.50 limit.

Thanks again, @Eric, for making it clear!

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