Analytical Design is all about realising that regardless of how much time, or how much money you spend on an initial web design, you can never be certain that your design is going to be the one that sells well.
Nor can you be certain that it will still work in 6 months or 12 months time. Sure, there are principles that a good designer knows will assist with sales, but each customer profile will respond differently. Each brand will get a different result, and each culture will buy based on different reasons. Even the time of year will affect the effectiveness of different designs.
Mostly, changes in a design are all about satisfying the business that the same design that they have had for years needs a re-fresh. While this sounds like sound advice, and may even be part of the reason for a change, the real reason should be based on metrics.
The business' online manager should have so much data - empirical evidence - that the site is under performing, that a re-design is needed.
Depending on your current results, this might be some minor A/B testing or Multivariate testing, of individual elements or it could require wholesale changes to the navigation, and the structure of information on your site.
As the results improve, testing and measuring results becomes more refined, and deeper within the site, but it should never stop. Just like the operation of a business, monitoring the internal and external factors, compared to the bottom line can mean the difference of continued growth business, and a flat line or decline in business.
Analytical design is building and testing as many different designs as it takes to get the results you are happy with, then continuing to test further changes to improve your results.
So next time you hear that 'the new design is finished' you'll know the real work should be just beginning.