"Where should I run this campaign?"
"Which outlet will provide me more ROI on a campaign?"
That's not the designer's job to determine.
Truth of the matter is the sales team, the order takers, the accounting department, heck even possibly the shipping department, will all have a better handle on where orders are received in a more amenable fashion and where more orders are coming from. Then factor in actual marketing departments whose entire basis is statistical analysis of customer information and it becomes clear that the designer is among the least qualified to determine where a campaign will have the most success.
I'm running a campaign on [VenueA] and [VenueB]. What will work best?
This is the designer's job. The designer should be aware that a campaign seen on [VenueA] will primarily be seen by a younger audience, where as a campaign on [VenueB] will be viewed by an older audience. Then be able to alter the design of the same content to match aesthetics better suited for a given demographic.
At times this demographic data is somewhat inherent in the venue itself. When dealing with Facebook, Twitter, Instagram, et al. It's fairly understood that the largest demographic is under 40. Compared to dealing with newspapers, whose demographic is traditionally older.
While no designer needs to scour over demographic and marketing data to any great degree, it is possible to get a cursory understanding of a demographic via a simple web search for the data.
Most web-based companies will have the data kind of readily apparent somewhere. So searching for "Facebook demographics" can easily lead you to a quality link.
When dealing with non-web-based companies things get much more difficult because it's rare or impossible to find such data for non-web companies, unless it's provided by a marketing department. It's often guarded with much more secrecy. However, asking the client "who their customer is" will provide some general information on their demographics. Sometimes this is all the designer has to base thing on.
Marketing is a profession unto itself for a reason.... it takes experience, time, and effort. While understanding what works and does not work for general demographic ranges should be important to a designer. The designer should not be dictating to the client where a campaign will work best. The designer simply won't have all that information or the expertise to sort through the information if they do happen to get it.
Now, as far as what demographics mean in terms of design alteration... well, that's a manual or book unto itself. Part of a good design education or experience is the knowledge with respect to the variances common when looking at different demographics. Things like type sizes based upon age, color trends based upon age/affluence, etc. are all general knowledge every designer should have. There are many questions on this site dealing with these specific topics:
There's also the "trend" factor which is a bit undefinable. To me, "trends" start by someone copying something they liked, then someone else likes it and copies it again, then it's copied again, and again, and again until it's a "trend". There's is often no rhyme or reason to trends. So, really the only way to keep up to date on trends is to observe the world around you. Sites such as this one are great at seeing what's being asked and talked about. But you probably won't find any hard, statistical, data on trends anywhere - at least not until after the trend is long dead.
I realize some companies throw everything into a hat and call that a "design" position. Marketing is one of those areas that is tangent to design and can be thrust upon a designer by companies at times. It is possible to wear multiple hats if that's your desire and be good at marketing+design. But that's not generally the "norm" for a designer. In broad terms, designers aren't marketing experts. Much like sales copy writing, the designer will almost always do a far inferior job at marketing than actual marketing professionals, the same way marketing professionals will always do an inferior job at designing than the design professionals.
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