OCRA – Multitask

The IBV as reference center has as its maximum the improvement of its products and services considering the current regulations as well as the improvements and advances that are produced from the scientific and technical point of view.

For some time now, researchers and clients have been pointing out "wrong" behavior in calculating the OCRA Index for certain very specific cases.

This behavior affected mainly the cases of analysis where it was considered more than one sub-task. So far, and as stated in UNE EN 1005-5 (2007), the OCRA index was calculated as the quotient between ATA and RTA, so that when there was more than one sub-task, it performed an averaging. This averaging favored incoherent results in some cases when several subtasks were considered in the analysis

The authors of the OCRA method itself, had also noticed this aspect, which they collected in the article "Ocra method: a new procedure for analyzing multiple repetitive tasks" [E. Occhipinti, D. Colombini, 2009]

For this reason, and following the indications of the authors of the OCRA method, in version 17 of Ergo/IBV, the calculation of the OCRA index is no longer obtained as an average, but rather the following formula is used to obtain it:

 

OCRA Multitask = OCRA1 (DuM1) + (ΔOCRA1 x K)

where:

  • 1,2,…,N Repetitive subtasks ordered by the values of the OCRA index (1 = the highest, N = the lowest) calculated considering the multipliers of the real duration (DuMi) and RcM (the same for all the tasks).
  • DuMi Duration multiplier considering the actual duration of sub-task i.
  • DuM Duration multiplier considering the total duration of all repetitive subtasks.
  • ΔOCRA1 OCRA of sub-task 1 considering DuM, minus OCRA of sub-task 1 considering DuM1.

    K= (OCRA1max x FT1) + (OCRA2max x FT2) + … + (OCRANmax x FTN)OCRA1max

  • OCRAimax Score of sub-task i considering DuM.
  • FTi Time fraction of sub-task i (value from 0 to 1) with respect to the total time that the repetitive task lasts.

The current calculation method for the OCRA index follows the same fundamentals used by the NIOSH equation to assess the risk of manual handling of loads in multiple tasks. It takes the initial index of the highest risk sub-task and adds increments in an orderly manner for the other sub-tasks.

This change only affects the final calculation of the OCRA index, and is not reflected in the calculation of ATA and RTA, which are calculated in the same way:

ATA is obtained by the following equation:

ATA=j=1n[Fj x Dj]

where:

  • n Number of repetitive subtasks performed during the working day
  • j Generic repetitive subtask
  • Fj Frequency (technical actions / min) of sub-task j
  • Dj Duration (minutes) of sub-task j

And RTA is obtained by the following equation:

RTA= j=1n[CF x Dj x ( FoMj x PoMj x ReMj x AdMj x DuMj)] x RcM

where:

  • n Number of repetitive subtasks performed during the working day
  • j Generic repetitive subtask
  • CF Frequency constant = 30
  • Dj Duration (minutes) of sub-task j
  • FoMj Force multiplier of sub-task j
  • PoMj Posture multiplier of sub-task j
  • ReMj Repetitiveness multipliers of sub-task j
  • AdMj Additional multiplier of sub-task j
  • DuMj Duration multiplier of sub-task j
  • RcM Recovery multiplier

The new formula for obtaining the OCRA index solves the calculation problem when several subtasks are considered in an analysis, as well as allowing a reliable analysis of rotations between different tasks analyzed with OCRA.