THE OCRA INDEX METHOD FOR DETAILED RISK ASSESSMENT

THE OCRA INDEX METHOD FOR DETAILED RISK ASSESSMENT

1. OCRA index computation

The OCRA index is the ratio between the Actual number of Technical Actions carried out during the work shift (ATA) and the Reference number of Technical Actions (RTA) (for each upper limb) which is specifically determined in the examined scenario [11, 38].In practice:

The assessment procedure involves taking the following three basic steps:

Step 1. Calculate the frequency of technical actions/min and the overall number of Actual Technical Actions (ATA) carried out in the shift (by each upper limb).

Step 2 Calculate the overall number of Reference Technical Actions within a shift (RTA).

Step 3 OCRA index calculation and risk evaluation.

Step 1

Calculate the frequency of technical actions/min and the overall number of Actual Technical Actions (ATA) carried out in the shift (by each upper limb).

First, identify and count the technical actions in a representative cycle of each repetitive task in the job.

The ANNEX 1 describes how to determine the technical actions:

Then use the following procedures and formulas to calculate the frequency of technical actions/min and the overall number of Actual Technical Actions (ATA) carried out in the shift (by each upper limb):

a	count the Number of Technical action in a Cycle (NTC)
b	evalutate their Frequency per minute (F) using the Cycle Time in seconds (CT)
F= *NTC* x 60 / CT (1)
c	evaluate the net Duration of the repetitive task present in the shift in minutes
d	calculate the overall Actual number of Technical Actions actually carried out within the shift (ATA)
ATA =F x D (2)

EXAMPLE A1: how to determine the Net Duration of repetive task and cycle time
EXAMPLE A1: how to determine technical action frequency and ATA

Step 2

Calculate the overall number of reference technical actions within a shift (RTA).

The OCRA method, when computing the number of reference technical actions, considers several risk factors and corresponding multipliers. The following general formula calculates the overall number of Reference Technical Actions within a shift (RTA)

30	“Constant of Frequency” of technical actions per min = 30;
X
Fo_M	force multiplier
X
Po_M	posture multiplier
X
Re_M	repetitiveness multiplier
X
Ad_M	additional multiplier
X
D	net duration (in minutes) of the repetitive task
=
RPA	Partial Reference number of technical Actions for task
X
Du_M	duration multiplier
X
Rc_M	recovery multiplier
=
RTA	the overall number of Reference Technical Actions within a shift

*RTA =CF* ´ Fo_M ´ Po_M´ Re_M ´ Ad_M ´ D´ Rc_M ´ Du_M (4)

More in detail consider the following sub-steps:

Step 2.1 Start from CF = 30

Step 2.2 Determine the Force Multiplier (Fo_M )

The force multiplier, FoM, is 1 if the following “optimal” conditions (see EN 1005-3) are met:

a) the isometric force does not exceed 50 % of the values proposed for 15^th force percentile for professional use in the healthy adult European population;

b) actions do not imply fast movements;

c) the frequency of force exertions is no more than 1 in 5 minutes and the action time is no more than 3 seconds;

d) the duration of the repetitive task is no more than one hour.

If these conditions are not met, use Table 1 to determine the force multiplier (FoM) that applies to the average level of force, as a function of time. The force level (upper row) is given as a percentage of Maximum Voluntary Contraction (MVC) or as a percentage of the Maximal Isometric Force (Fb) as determined in EN 1005-3 (Step A). If the percentage of MVC or the Fb are difficult to assess, a value derived from the application of the CR-10 Borg-scale [6, 7] can be used (second row). The corresponding Force Multiplier (FoM) can be derived from the table. Use a FoM = 0,01 when the technical actions require 'peaks' above 50 % of force or a score of 5 (or more) in Borg-scale for almost 10 % of the cycle time.

The Annex 2 explaines how to determine the force level.

The values in the Table 1 can be interpolated if intermediate results are obtained.

Force level in % of MVC or F_b	5	10	20	30	40	≥ 50
CR-10 Borg Score	0,5	1	2	3	4	≥ 5
CR-10 Borg Score	very, very weak	very weak	weak	moderate	somewhat strong	strong/very strong
Force multiplier (Fo_M)	1	0,85	0,65	0,35	0,2	0,01

Table 1 — Multiplier relative to the different use of force

The EXAMPLE A1 describes how to determine the FORCE MULTIPLIER

Step 2.3. Determine the Posture (and movements) Multiplier (Po_M)

The multiplier PoM is equals to 1 when one of the postures or movements, described in Table 2, is present for less than 1/3 of the cycle time: otherwise use Table 2 to obtain the specific multiplier factor. Choose the lowest multiplier PoM (that corresponds to the worst condition) between the posture and movements evaluated.

Also consider shoulder postures and movements by checking that:

1.the arms are not held or moved at about shoulder level (flexion or abduction at about 80° or more) for more than 10 % of cycle time and/or for more than 2 actions per minute [42];

2.the arms are not held or moved in mild abduction (between 45° and 80°) for more than 1/3 of cycle time and/or for more than 10 actions/min.

3.the arms are not held or moved in mild extension (more than 20°) for more than 1/3 of cycle time and/or for more than 10 actions/min.

If one of those two conditions occurs, a risk of shoulder disorders exists and should be accurately considered.

The Annex 3 explains how to analyse postures and movements of the upper limbs:

Main Awkward posture Multipliers[10]	less than 1/3 (from 1 % to 24 %)		1/3 (from 25 % to 50 %)		2/3 (from 51% to 80 %)		3/3 (more than 80 %)
ELBOW supination (³ 60°)	1		0,7		0,6		0,5
WRIST extension (³ 45°) or flexion (³ 45°)
HAND hook grip or palmar grip (wide span)
ELBOW pronation (³ 60°) or flexion/extension (³ 60°)	1		1		0,7		0,6
WRIST radio/ulnar deviation (³ 20°)
HAND pinch
SHOULDER flexion/abduction more than 80°
%`time	10	20		30		40		≥50
Multiplier	0,7	0,6		0,5		0,33		0,07

Table 2 ― Elements for the determination of the Posture multiplier (PoM)

The EXAMPLE A1 describes how to determine the POSTURE MULTIPLIER:

Step 2.4. Determine of LACK of Variations Multiplier (Re_M)

The presence of lack of variation (high repetitiveness or stereotypy ) of certain movements can be pinpointed by observing technical actions, or groups of technical actions.

Stereotypy means the presence of identical gestures (technical actions), repeated for at least 2/3 of cycle time (medium level ) or all the time (high level).

If cycle time is between 9-15 seconds, Repetitiveness must still be considered as present (medium level); if cycle time is equal or less or than 8 the level will be high (see Table 3 for determine the corresponding Multiplier)

LEVEL	(ReM)	DEFINITION
Absent	1
Moderate	0.85	PERFOMS WORKING GESTURES OF THE SAME TYPE INVOLVING SHOULDERS AND/OR ELBOW AND/OR WRIST AND/OR FINGERS FOR 2/3 OF TIME ( or cycle time between 8 and 15 seconds, full of technical actions performed by the upper limbs. These actions can be different from each other)
High	0.7	PERFOMS WORKING GESTURES OF THE SAME TYPE INVOLVING SHOULDERS AND/OR ELBOW AND/OR WRIST AND/OR FINGERS NEARLY ALL THE TIME ( or cycle time less than 8 seconds, full of technical actions performed by the upper limbs. These actions can be different from each other)

Table 3 ― Elements for the determination of the repetitiveness multiplier (ReM)

The EXAMPLE A1 describes how to determine the LACK OF VARIATIONS

Step 2.5. Determine of Additional Multiplier (Ad_M)

Besides the main risk factors (frequency and repetitiveness of technical actions, use of force, awkward postures and movements, lack of recovery periods, daily repetitive task duration), which are, examined elsewhere, there are others factors, of an occupational nature, that should be taken into consideration when exposure is assessed. They are defined here as additional risk factors. This is not because they are of secondary importance, but because each one of them can, from time to time, be present or absent in the contexts examined.

The list of these factors is not necessarily exhaustive and includes:

PHYSICAL FACTORS

vibrating tools tools with high level of vibrations

use of screwdrivers with countershock

other vibrating tools: choose the score considering the vibration level presented

precision tasks are carried out for over half the time (tasks over areas smaller than 2-3 mm)

the tools employed cause compressions of the skin (reddering, callosities, blisters, etc..)

use of glooves that interfere with the grasp

the working gestures required imply a countershock ( such as e.g., hammering, or hitting with a pick over hard surfaces, etc.) with frequency of 2 time per minute or more

the working gestures imply a countershock (using the hand as a tool) with frequency of 10 time per hour or more

exposure to cold or refrigeration (less than 0 degree) for over half the time

more than one additional factor is present at the same time and , overall, they occupy over half the time

ORGANISATIONAL FACTORS

working pace set by the machine, but there are “breathing spaces” (buffers) in which the working rhythm can either be slowed down or accelerated.

working pace completely determined by the machine

For each of the physical-mechanical risk factors, it is necessary to specify for how much time (as a portion of the cycle/task time like 1/3, 2/3, 3/3) the factor is present, or to describe the frequency of occurrence of actions where that factor is present (especially for sudden movements and movements with counter shocks). The assessment of additional risk factors begins with a definition of optimum conditions, as represented by the absence, or by the very limited presence, of additional risk factors: in this scenario the additional multiplier AdM equals 1. Any discrepancy with respect to this optimal condition represents a contribution of additional risk factors to the overall exposure level, which grows with the growing portion of the cycle time during which additional risk factors (one or more) are present. In those cases the additional factor AdM multiplier equals:

ADDITIONAL MULTIPLIER FOR PHYSICAL FACTORS

0,95 if one or more additional factors are present at the same time for 1/3 (from 25 % to 50 %) of the cycle time

0,90 if one or more additional factors are present at the same time for 2/3 (from 51 % to 80 %)of the cycle time

0,80 if one or more additional factors are present at the same time for 3/3 (more than 80 %) of the cycle time

ADDITIONAL MULTIPLIER FOR ORGANISATIONAL FACTORS

0,90 working pace set by the machine, but there are “breathing spaces” in which the working rhythm can either be slowed down

or accelerated.

0,85 working pace completely determined by the machine

Table 4 ― Elements for the determination of the Additional multiplier (AdM)

The Annex 4 explains how to analyse additional factors:

The EXAMPLE A1 describes how to determine the additional factors :

Step 2.6 Multiply the adjusted CF thus obtained for the net duration (in minutes) of the repetitive task (D_j) to obtain a Partial Reference number of technical Actions for task j (RPAj):

Step 2.7 Determine the Recovery period multiplier (Rc_M)

A recovery period is a period during which one or more muscle-tendon groups are basically at rest. The following can be considered as recovery periods:

breaks (official or non official) including the lunch break

visual control tasks

periods within the cycle that leave muscle groups totally at rest consecutively for at least 10 seconds almost

every few minutes.

For repetitive task the reference condition is represented by the presence, for each hour of repetitive task, of work breaks of at least 10 minutes consecutively or, for working periods lasting less than one hour, in a ratio of 5:1 between work time and recovery time . In relation to these reference criteria it is possible to consider how many hours, during the work shift, do not have an adequate recovery period. It requires the observation, one by one, of the single hours that make up a working shift: for each hour, a check must be made if there are repetitive tasks and if there are adequate recovery periods. For the hour preceding the lunch break (if present), and for the hour before the end of the shift, the recovery period is represented by these two events.On the basis of the presence or absence of adequate recovery periods within every hour of repetitive work, the number of hours with “no recovery“ is counted ( see Table 5 to define the specific Multipliers)

Number of hours without adequate recovery	0	1	2	3	4	5	6	7	8
Multiplier Rc_M	1	0.9	0.8	0.7	0.6	0.45	0.25	0.1	0

Table 5 ― Elements for the determination of the Recovery period multiplier (RcM)

The Annex 5 explains more in detail how to identify the number of hours without adequate recovery:

The EXAMPLE A1 describes how to determine RECOVERY MULTIPLIER:

Step 2.8 Determine the Duration multiplier (Du_M).

Within a working shift, the overall duration of manual repetitive tasks is important to determine the overall risk for upper limbs. When repetitive manual tasks last for a relevant part of the shift, the DuM is equal to 1. In some contexts, however, there may be differences with respect to this more “typical” scenario (e.g. regularly working over-time, part-time work, repetitive manual tasks for only a part of a shift); the multiplier (DuM) considers these changes with respect to usual exposure conditions. Table 6 gives the values of DuM in relation with the overall duration of manual repetitive tasks.

Total time (in minutes) devoted to repetitive tasks during shift	< 120		120 – 239		120 – 239		> 480
Duration Multiplier Du_M	2		1.5		1		0.5
Or to be more precise
Total time (in minutes) devoted to repetitive tasks during shift	< 121	121-180	181-240	241-300	301-360	361-420		421-480	> 480
Duration Multiplier Du_M	2.0	1.7	1.5	1.3	1.2	1.1		1.0	0.5

Table 6― Elements for the determination of the Duration multiplier (DuM)

The EXAMPLE A1 describes how to determine DURATION MULTIPLIE

Step 2.9 Evaluation of RTA (Reference Tehnical Actions) adjusting RPA_tot in relation to the recovery distribution and the daily duration (in minutes) of all repetitive tasks

Once RcM and DuM have been identified by Steps 2.7 and 2.8, the overall number of Reference Technical Actions within a shift (RTA) can be computed by the following formula:

RTA = RPA_tot ´ Rc_M ´ Du_M(6)

The EXAMPLE A1 describes how to determine RTA

Step 3

OCRA index calculation and risk evaluation

The OCRA Index is obtained by comparing, for each upper limb, the Actual number of Technical Actions carried out during the work shift (ATA) obtained in Step 1 and the Reference number of Technical Actions within a shift (RTA) resulted in Step 2, by means of this formula:

Use Table 7 to assess the risk and to decide for consequent actions to be taken

Table 7 ― OCRA Method : Final assessment criteria

It should be underlined that the OCRA index “critical values” reported in Table 7 should be used as an help to better frame the risk assessment and more effectively guide any consequent preventative actions, rather than rigid numbers splitting results between “risk” or “no risk”. For instance, though it is theoretically fair to state that an OCRA index value of 3.4 represents an uncertain risk and an OCRA value of 3.6 represents a definite risk, it is equally fair to say that the difference between these two values is negligible, and that users should pay due attention to trends in OCRA results (also using the forecasting methods supplied).

The specific ANNEX 6 gives details on the criteria adopted for OCRA index classification and about forecasting models of the expected percentage of Persons Affected (PA) by one or more Upper Limb Work Related MusculoSkeletal Disorders (UL-WMSDs):

The EXAMPLE A1 describes how to determine OCRA INDEX
Step 4. (NEWS) Ocra index: a new procedure for multitask analysis when rotation among repetitive tasks has a frequency of less than once per hour.
When computing the Ocra index considering the presence of more than one repetitive task, a “traditional” procedure has been already proposed. It is based on computing from one side the overall ATA [the sum of all the technical actions actually performed in different repetitive tasks during the shift by one upper limb] and from the other side RTA [the sum of all reference technical actions suggested for each task -RPAi – adjusted for multipliers concerning recovery periods (RcM) and total duration of repetitive tasks (DuM)]. This approach, which results could be defined as “average weighted for time”, seems to be appropriate when considering rotations among tasks that are performed very frequently, for instance almost once every hour (or for shorter periods); in those scenarios, in fact, “high” exposures are presumed to be in some way compensated by “low” exposures that alternate very quickly each other. As a consequence of this statement the traditional procedure for Ocra index multitask analysis is confirmed when rotation among repetitive tasks is performed almost every hour or when the single tasks are really sub-tasks of a general “complex” task (whose cycle time generally lasts several minutes). The index will be defined Ocra Index Multitask Average.
The EXAMPLE MULTITASK A describes how to determine OCRA INDEX multitasks with job rotation lasting no more than 1 hour
On the contrary, when rotation among repetitive tasks is less frequent (i.e. once every 1,5 or more hours), the “average weighted for time” approach could result in an underestimation of the exposure level (as it practically flats peaks of high exposures). For those scenarios an alternative approach based on the “ most stressful task as minimum” could be more realistic. The result of this approach will be, as minimum, equal to the Ocra index of the most stressful task considered for its individual duration and ,as maximum, equal to the Ocra index of the same most stressful task when it is (only theoretically) considered as lasting for the overall duration of all examined repetitive tasks. A peculiar procedure allows to exactly estimate the resulting index within this range of minimum to maximum values. The consequent index will be defined Ocra Index Multitask Complex.This later approach has been already settled out in the NIOSH approach for multiple lifting tasks and, given the recent availability, in the Ocra computation procedure (RTA computation), of more detailed duration multipliers (practically one different DuM for each different step of one hour of duration of repetitive task), it becomes possible to define a peculiar procedure to compute the Ocra Index Multitask Complex for the analysis of two or more repetitive tasks when rotations are unfrequent (rotations every 1,5 hours or more).
The EXAMPLE MULTITASK B describes how to determine OCRA INDEX multitasks with job rotation lasting more than 1 hour

2. OCRA index: risk reduction

Example A2: risk reduction optimising breaks distribution

We can use different solutions to reduce the risk evaluated in Example A1.

Reducing the number of cycles and increasing consequently the cycle time, means proposing to significantly reduce the production: this is the last way to use for risk reduction. One suggestion is to re-arrange the breaks distribution, considering the possibility to optimising the recovery periods. In the example A 1 there are a lunch break and two breaks of 15 minutes each, one before and the other after the lunch break (in the last hour of the shift). The number of hours with

“no recovery“ is in this case 5 (one of the 2 breaks is in the last hour of the shift in which a recovery is already considered, as represented by the end of the shift). It is possible to obtain a significant risk reduction simply dividing the 30 minutes of breaks in 3 breaks of 10 min. each and correctly distributing them in the shift (Table 8).

The OCRA index consequently shifts in the yellow zone.

This example shows that in some situation the only optimisation of recovery distribution can be sufficient for obtaining a riskreduction without any cost.
The forecast of PA (% of Person Affected by UL WMSDs) decreases as showed in the following Table 9

The EXAMPLE A2 describes how to reduce the OCRA index optimising the break distributions

Example A3: risk reduction improving postures

To improve the results obtained in Example A2, one can imagine to improve the wokplace lay –out. As showed in the movie a tapis-roulant leaves the first pieces at the left side of the worker. In re-designing this workplace could be useful to stop the tapis-roulant closer the worker (simple and cheap solution), and to train the worker for a better way for assembling the 2 pieces.

He has to take the first piece from his left side with the left hand instead with the right and consequently the second piece with the right hand. Taking and positioning both with this strategy, the worker can avoid to maintaining the pieces in his hand reducing consequently the % time in pinch posture). The Posture multipliers (PoM) for the right upper limb will be now

Hand in pinch for only 40 % of the cycle time PoM = 1 for right and left

The OCRA index consequently shifts in the green zone.

OCRA index (left) = 2				OCRA index (right ) = 2,1
Forecast of WMSDs	min	AVERAGE	max	min	AVERAGE	max
	4.4	4.9	5.4	4.5	5	5.5

Table 10

The forecast of PA (% of Person Affected by UL WMSDs) decreases as showed in the following

Table 10:

The forecast of PA is now similar to the one found in a reference group of workers not exposed to repetitive tasks

The EXAMPLE A3 describes how to reduce the OCRA index improving the lay-out

Example A4: risk reduction adding the possibilty to partially modulate the machinery pace.

Adding at the machinery “breathing spaces” in which the working rhythm can either be slowed down or accelerated,

the corresponding Multiplier for the organisational additional factors shifts from 0,85 to 0,90

The OCRA index consequently shifts in a full green zone.

OCRA index (left) = 1,9

OCRA index (right ) = 2

The EXAMPLE A4 describes how to reduce the OCRA index improving the WORK RHYTHM

Example A5: increase the productivity

The techicians can try now to increase the productivity maintaining an acceptable exposure level.

Using the software midaOCRA, increase the number of pieces and observe the OCRA index variation.

Increasing the number of pieces from 5220 to 5800 (+11%), the OCRA value stays in green area.

OCRA index (left) = 2,2

OCRA index (right ) = 2,25

The EXAMPLE A5 describes the variation of OCRA index, increasing the productivity

Increasing the number of pieces from 5220 to 6000 (+15%), the OCRA value stays in yellow area (bordenline risk)

OCRA index (left) = 3,3

OCRA index (right ) = 3,4

The EXAMPLE A6 describes the variation of the OCRA index increasing more the productivity