How do I measure the effectiveness of an online ad?

The logical thing to do is to check the number of viewers and then compare it to the feedback or response.
Do a statistic of the sales before the ad, and then after it.

Best Part of online advertising is that it is purely a ROI ( Return on Investment ) driven medium to advertise.
The effectiveness of an online ad is measured in terms of CTR meaning click through ratio.
CTR = (No.
of Clicks / Total number of Impression Served ) X 1000 As per industry standard a good CTR is considered to be in between 0.1 to 0.
3 it means out of 1000 impression of ads served on any website the ad should get at least 1 to 3 clicks.
Normally we use ZEDO or Mediamind tags to track clicks on any online ad.
Some organization use CPL & CPA to check effectiveness.
CPL stands for Cost per Lead & CPA Stands for Cost per aquisition.
If you want to have more details any one of you can get in touch with me on faizan2020@GMAIL.COM

في البداية يجب علينا أن نحدد نسبة المبيعات للمنبج قبل بدء الاعلان، و من ثم عند الحملة نبدأ بالقياس حسب أيام العرض و طريقته و مكانه و إمكانية وصول المنتج للمستهلك، و من ثم نقيس النسبة بعد انتهاء الاعلان و عندها يمكن لنا أن نحصل على نسبة و مع ذلك لا تكون دقيقة، لان قرار التعامل مع المنتج قد يكون غير لحظي و إنما تراكمي ..
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Web advertising is the fastest growing advertising medium.
Web advertising, unlike its print, radio, or television counterparts, is easily measured and tracked using basic software applications.
You can measure the effectiveness of Web advertising by utilizing marketing analysis software to gather online statistics.

Trace the ad with a click counter.
The more clicks, the more apealing the online ad is.

put a tracker on your online ads and use analytics tools like Google Analytics.
I prefer to use URL shortening tools like Bitly for example as they track every single click, unlike GA which omits a lot of clicks.
Rotate your ads and try different variations.
See which one generate the highest CTR (Click Through Rate) and which one actually convert your visitors into customers.

I agree with Omar Haidar

i think understanding the ad goal is important , why you post it , increase the sales ? increase website traffic ? build brand awareness ? then you will know what to use, i.e your goal is increasing website traffic then google analytics is your major tool , the tool will give you reports when people visit your site , how they comes to your site ? through your ads or not our they guided by search engines and what keywords that they use to search for your site ......
etc

Google Analytics is the solution. Kindly Make an account, make proper funnels. The link you provide  in yor online add make it sure it has the specific utm source so you can check from where the traffic is coming from. So whenever the customer clicks your add it will show that specific utm source. Utm codes are tracked in Google Analytics.

By gauging if it helps you achieve your intended goal (sales increase, repositioning etc)