Tracking offline conversion is tricky, but here are some potential strategies to measure the effect of an online ad on physical store sales:
Build a discount voucher system
Create a coupon code system to track the source of sales. This could be a simple offer code that visitors quote at the physical checkout, or a voucher to print. Here's a system to minimise the chance of friends sharing the code so you measure only people who've clicked the ad:
- The customer clicks on the ad, which uses a link including a query string such as
http://yoursite.com/yourpage?source=campaignname
- Some code in the head of the product page detects the presence of the
source query string, sets a cookie called 'source' containing the source information, and redirects immediately to:
http://yoursite.com/yourpage
- Some code on the product page detects the presence of the cookie, and shows an HTML voucher with the following text:
"Get 10% off [the product name]. Print this voucher and present it at [store name or address]. (For merchant use only: REDEEM23A)".
This redirection system ensures that, should the user pass the link on to a friend, their friend sees no offer code (because the query string has been stripped from the URL, and the friend won't have the 'source' cookie). There's nothing to stop someone printing multiple vouchers and handing them around, of course, but it still makes the barrier a little higher.
Improve it using unique offer codes
If you wanted even more control over the offer redemption, in step 3 above, you could generate a unique code, store it in a database, and put that offer code on the voucher. Then, at the end of each month, collect up all the vouchers and check the offer codes against the ones in your database to rule out duplicate codes and build a more accurate picture of who used your vouchers.
In addition, the section that appears for customers who click the ad could prompt them for their email address and send them a unique voucher code. They might be more likely to print the voucher if its stored safely in an email.
I appreciate that your client might not be too keen to offer 10% off their products, but you could reassure them that it will likely save them money; offering a small discount will help them track the success of their campaigns to ensure that they're not wasting money on ineffective marketing.
Target the ad at one geographic location only
If your customer has multiple stores, you could run the ad only for visitors from a certain area and see if sales in that area pick up. Facebook and Google AdWords both offer geo targeting of ads.
Run only one advert
If you can't geo target your ads for any reason, you could target by time instead. Run only that ad in a given week in which sales volume is generally stable and well known, and measure effect on sales.
Turn it on and off
Run the ad for four weeks, and turn it off for four weeks and measure the difference against the usual baseline of sales for each four-week period.
Don't run online ad campaigns back-to-back
Have a small gap between online ad campaigns to see if sales return to the baseline in between campaigns, and to better gauge the difference between two ads you're trialling.
If measuring phone sales, use a different number
Detect traffic from the ad using the method described above, then show a different phone number on your page if you'd like to gauge the impact on phone sales.
In summary
It's very hard to do accurate cost-benefit analysis for online ads when you're measuring conversion rate offline, but using a custom voucher code system and carefully limiting the location, frequency, and number of ads you run at one time are among the best methods to measure ad effectiveness.
