The Michael David Winery is perhaps the best-known wine producer in Lodi, California. It has recently been in the news, as it was named the 2015 "Winery of the Year" by the Unified Wine & Grape Symposium; and its Seven Deadly Zins old-vine zinfandel topped Shanken’s Impact newsletter's “Hot Brand” award list in both 2015 and 2016. Not bad for a Central Valley winery producing 500,000 cases per year, with more than two dozen different wine labels.
The Wine Industry Advisor notes for the Seven Deadly Zins (a blend of grapes from seven Lodi growers) that: "As sales grew so did the scores, as it achieved 90+ ratings from both Robert Parker and Wine Enthusiast Magazine over the past three vintages." Indeed, the first graph shows the Wine Enthusiast scores for eight vintages of seven different wines from the Michael David Winery. With two exceptions (in 2008), the scores do seem to increase through the vintages.
Is this pattern real? Obviously, it depends on what we mean by "real", but in this case it might best be interpreted as: do other wine assessors agree that there has been an increase in quality? There are not many of these to choose from, as most wine critics ignore Lodi wines, preferring instead to look further west when tasting California wines. However, Cellar Tracker is more reliable in this regard, since its scores come from wine drinkers rather than professional wine tasters.
So, this next graph shows the Cellar Tracker scores for exactly the same wines and vintages. I do not see any general time trend here, although a couple of the wines do increase for their most recent vintage. That is, the scores are very consistent except for one vintage of Seven Deadly Zins and one of Earthquake Petite Sirah.
So, what is going on here? Why the discrepancy between the Wine Enthusiast and Cellar Tracker? The answer appears when we look at who actually provided the Wine Enthusiast scores.
To show this, I have simply averaged both the Wine Enthusiast and Cellar Tracker scores across the available wines for each vintage, thus producing a consensus score for the winery through time. These averages are shown in the next graph, for both sources of wine scores. Note that the Cellar Tracker scores show the Michael David Winery to be a consistent 88-point winery, with only a slight increase in score through time.
However, the Wine Enthusiast scores are another matter entirely. At least three different reviewers have provided the scores, as shown at the bottom of the graph; and it is clear that these people have very different scoring systems. This is explained in more detail in the post on How many 100-point wine-quality scales are there?
So, in this case the apparent increase in Wine Enthusiast scores through time is illusory. It just so happens that, through time, the three reviewers have increasing tendencies to give high wine scores, while the people using Cellar Tracker do not. The quality of the Michael David wines is not in doubt, but what scores their wines should be given is clearly not yet settled.
This is a general issue that we should be wary of when studying time trends. We first need to ensure that the data are comparable between the different years, before we start comparing those years.
Thanks to Bob Henry for first bringing the Michael David wines to my attention. Incidentally, my favorite of these wines is the Petite Petit, a ripe-fruited blend of petite sirah and petit verdot, which is good value for money.