Sum up durations (rollup) from looked up data (from multiple records) into one cell

[oVo_Team]

Hello, i'my trying to compare efficiency of real-time project durations to theoric template durations. 

Projects are linked to tasks and tasks templates ;

-> total duration of a project = sum of the duration of each task

-> duration objective (efficiency) of a project = sum of the task duration objectives

In each project, Im trying to lookup the tasks theoric duration and sum it up to compare it to the actual project duration. 

I'm doing this this way : 
- I link projects to tasks

- Each task is linked to a ''task template''

- I look up the ''theoric duration'' of each task and Sum it up in rollup

The problem is that the sumup calculation is incorrect : see screenshot ->

- the first row, lookup values for theoric durations per project are as follow : 10, 10, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8

=> And the calculated Sum is 20...

Does anyone encounters problem with this kind of sumup ?

Thanks for your help

[oVo_Team]

 Tried so but still not working 😕 It seems the issues comes from the fact that the original field is a lookup (rollup from a lookup). Is it possible that a number data stored in a lookup field changes to a string ? How to change it back to a number ?

[Sho]

Oops, value(values) is an error.
Try sum(values)

[Databaser]

By using a rollup field instead of a lookup field for that original field. 

[oVo_Team]

Oh OK guys i think i almost got it ! it seems airtable rollup field switched to 0 the data when the lookup field has multiple values, see screenshot attached 

Lookup = 8, 8 ===> Rollup = 0

Lookup = 10 ====> Rollup = 10

Lookup = empty ====> Rollup = 0

Considering this, how do you thin i should solve this ?

[Sho]

@Databaser is right, the original Lookup field also needs to be a Rollup field.