How do I Measure SEM Effectiveness?

How you measure the effectiveness of your search engine marketing program depends on what your objectives were when you started. If your intent was merely to increase the number of visitors to your Web site, you would determine if your SEM efforts were successful by comparing your Web traffic statistics before and after you implemented search engine marketing.
Be patient though, search engine marketing efforts may take considerable time to boost your traffic. Although you can instantly get your PPC ads running, if you are aiming at improving traffic through SEO, it can be several months before you see solid results - especially through Google. Likewise, if you request a directory listing from DMOZ, your listing may not appear for 6 months to a year. DMOZ is staffed by volunteers and, because DMOZ listings can have a positive affect on your organic search rankings, it is arguably the most popular and valuable free Web directory listing to get.
In addition to site traffic, you need to track your conversion rates. For example, if, as a result of your search engine marketing efforts, your traffic doubles from 250 to 500 visitors per month, how many new customers did you acquire from the additional 250 visitors to your site. Do you now have twice as many customers as you did before? Probably not. If you picked up 5 customers your conversion rate would be 2 percent of the new traffic (5 divided by 250) and 1 percent (5 of 500) overall.
Find ways to increase the number of visitors to your Web site, and then find ways to increase the conversion rates of those visitors, and you'll know exactly what it takes to make your search engine marketing program successful.