How we can measure effectivness of the controls measures selected.and how we can measure the contribution of each selected control in reducing risk?

Effectiveness of the control measures are defined base on the statistic of the recurrence of accidents. This is the reason why in any management  system you have the internal audit to assess the effectiveness of the identified control measure. Also in PDCA you have the C part which is check to determine if your control measure is effective or not. If it is not effective then identify another control measure.

the purpose of control measures is to reduce incidents at work. control measures are adopted by a team work discusions to determine the right controls for the case. worker participation is very important to effectively adopt the right controls needed to prevent incidents.1st a risk assessment is done for the case, 2nd a plan is made to come up with the right controls 3rd worker participation will be required to discuss the correct controls measures, 4th controls are adopted and reviewed, 5th controls are put in use. monitoring effectiveness of adopted controls will follow by periodic inspection with check lists, auditing will be conducted to determine gaps arising (if any) and reccommend actions to be taken.

THROUGH WALK THROUGH, INSPECTIONS, SITE VISITS, RISK ASSESMENTS , AUDITS , JOURNELS AND MEDIA ETC....