Isn't Webmetrics affected by Internet traffic, which would impact the performance data I receive from you?
Everybody on the Internet is on a shared network. You may have the biggest pipe in your neighborhood, but at one point or another you are connected to a shared network.
Our servers are affected by this just like everyone else, including potential users of your site. When you are analyzing your performance data, look for trends rather than particular instances. For example, say a customer is monitoring a five-step e-commerce process. Looking at the logs, they notice that the third page in the scripted transaction takes approximately 10 times as long as the other pages to download, but the page is about the same size as the others. On further investigation, the customer discovers a problem with their database which was affecting the third page's load time, which they can then address.
Sometimes, spikiness in graphs or increased average load times can occur in monitoring data due to network latency from agents that are geographically distant from where your site or application is hosted. You can set up baseline agents to reduce this effect. Overall, performance data is generated to give you a general idea that things are functioning well, and when looking at trends, the data should make it clear when there is a problem.