This argument is a good example of a
critic selecting a particular time frame to defame those guests by selecting a very specific 12 year period where their recommendations underperformed than the market. We can weaken this argument by presenting evidences where their recommendations were good in terms of performance, any difference in methodology between market and stocks performance calculation, etc.
(A) Taken together, the stocks recommended on the television show performed better than the market as a whole for the past year.
This is a
Weakener, out of 12 years, there was a year when stocks performed better, this slightly reduces our confidence in the conclusion.
(B) Taken together, the stocks recommended on the television show performed better for the past 12-year period than stock portfolios that were actually selected by any other means.
This is a
Weakener, if those stocks' recommendations performed better than any other then we do have a reason to believe in them because at least they gave us better returns than other portfolio managers or stocks research analysts.
(C) Performance of the stocks recommended on the television show was measured by stock dividends, whereas the performance of the market as a whole was measured by change in share value.
This is a
Weakener, this is more on the line of apple vs. orange comparison. In a simple case, A travelled 10km and B travelled 100m, then someone comes and says B travelled more than A which is very illogical.
(D) Performance of the stocks recommended on the television show was measured independently by a number of analysts, and the results of the all the measurements concurred.
This is a
Strengthener, this explains how the analysis was done in a proper way and despite that, their recommendations performed worse than market so we have a reason now to not follow their recommendation. Furthermore, this option tells us that maybe their analysis is not good so we shouldn't follow their advice.
(E) The stock portfolios for which the guests were consultants performed better for the past 12-year period than the market as a whole.
This is a
Weakener, this says their portfolios performed better than previous 12 year period which does imply that their recommendations can be good and we can follow their advice.