how can you measure the effectiveness of a marketing campaign or strategy?

Answer 1

By identifying the tangible results and impact among the target crowd (whom the campaign was aimed at)
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Appropriate Marketing research methods such as ,Questionnaire survey,Focus groups,Recall studies ,etc can be used depending upon the nature of the campaign. The research findings are usually validated by comparing with the performance of the product ( service) in the market in the post campaign period. hope this helps

Answer 2

Coupons. Free gift offers. Run an ad that says bring in this coupon and we will give you $1.00 off. Then count the people with coupons who respond.