The key principles of effective design
Communication is above all the forefront of effective design. Graphic Design by nature is about communicating an idea. Regardless of medium this is the de facto standard for an effective design.
Quick Historical Look:
Nearly 200 years ago William Playfair (1786) began the serious use of graphs for looking at data. More than 50 years ago a battle raged on the pages of the Journal of the American Statistical Association about the relative merits of bar charts and pie charts (Eells 1926; Croxton 1927; von Huhn 1927). Today graphs are a vital part of statistical data analysis and a vital part of communication in science and technology, business, education, and the mass media.
Source: Graphical perception: Theory, experimentation, and application to the development of graphical methods in the Journal of the American Statistical Association, Vol. 79, No. 387. (Sep., 1984), pp. 531-554.
Larkin and Simon (1987) suggest that the following forms of cognitive efficiency are offered by good graphical displays.
Substituting Visual Operators: Graphical displays often allow users to substitute less demanding visual operators in place of more complex logical operators in place of more complex logical operators. Visual operators (e.g., distance and color comparisons, spatial coincidence judgements) can often give users the same information as more complex non-visual operators.
Reducing Search: Graphical displays often arrange information in such a way as to reduce the number of items the user must look at in order to find something useful, or to group information required to draw a particular inference into one spatial locality. Graphical techniques like shading and spatial arrangement can help guide the eye to relevant information or past irrelevant information.
A lot of designers especially today get overshadowed by creativity and can get too far from the initial goal of communicating a goal. Here's a passage from Visual Design Principles for Usable Interfaces (Watzman, Suzanne) that I think is very relevant:
Defining Visual Design. Visual design is not merely a series of subjective choices based on favorite colors or trendy typefaces--at best a cosmetic afterthought considered if there is enough time and money. Good visual design is the tangible representation of product goals. It is concerned with the "look," the method, and the style in which the information is presented. It should be the result of a thoughtful, well-considered process, not merely a decorative afterthought.
Applying the appropriate visual/experience design principles and tools while incorporating the user perspective (information design) enhances the value, perception, and usefulness of products. It is the best combination of project goals, the user perspective, and informed decision making.
Design is in direct relationship with the economy. When we design for Engineering it is very easy to see how the design solution must meet certain economic goals. In architecture and graphic design this is often not the case. They can become ornate and saturated, especially when the economy is good. The budget is there. When the economy is bad then things get simplified, mass produced, reusable, templates and frameworks --- all of it. There are design solutions that are possible within these parameters and they reduce the overall cost of goods. When times are good, or you get lucky enough to land a client with the money to spend then its easier to justify new paradigms and reasons.
Reducing Search: Where the Principles of Design come in
If the goal is to communicate an idea then to best do this, we as designers, must have some clue how people (unless you're designing for other species) perceive the world around us.
Top-Down Processing. Widely seen as the bulk of our processing capabilities is that we start at the larger entities before working our way down into the details. In this way our expectations affect our perceptions. For examle you probably filled in the letter P just now, because your brain expected it to be there. There are a number of tests you can look at including, The Hollow Head, which tricks your brain into seeing what isn't there. It does this because of top-down processing giving us expectations about what SHOULD be there, rather than what IS there. (An immediate example of this in the traditional realm of Graphic Design would be the cover of Invisible Monsters by Chuck Palahniuk:
Bottom-up Processing. Also called data-driven processing, is when you have no expectations. You have to start small and work to fill in holes until you find an expectation.
Additional Study: Khan Academy Lesson: Bottom-up vs Top-down Processing
As a Graphic Designer you can think of this as the "aha" moment. If the majority of your intended audience can immediately identify the whole then they use almost all Top-Down Processing. This is a key ingredient for a successful design. It requires less searching from one spot to another in the hopes of figuring out what is going on.
I'm going to end this here, at least for now. At this point you can get into the more technical stuff like the Gestalt principles that Dominic has begun to discuss.
Application / Implementation
I realize some of this stuff seems fairly complex, particularly for new designers to grasp. Over the next few days I'm going to add images in this section but wanted to at least get some basic stuff in writing to help people out.
First Iteration of an Advertisement or Website:
On your first iteration of an advertisement pretend you put in a picture of your product in use. Pretend for now its a water bottle being drunk by someone on a crowded basketball court.
You look at it and think, wow this is a great shot of our product! You show your coworkers and they think, wow our designer did a great job showing our product! You show the rest of the world though and you know what they're going to think? What's the product?
They'll have to search the page for clues.
To begin fixing this you might think, how can I make this clear. People should be able to quickly scan the ad and know exactly what it means. It should be as much top-down processing as possible.
So perhaps the first step is adding a logo. Put a logo on the bottom right and the logo is visible on the water bottle. That's a good clue. Is it enough though? Probably not.
You work on the copy and add some large text, "Always cold water! The world's first water bottle with solar panels that can keep you hydrated on the hottest days!"
Alright, we're making progress. Now someone sees a photo, identifies the logo, and discovers from the copy that we're talking about some sort of water bottle.
Circle the water bottle. Darken everything but the water bottle. Oversaturate it. Do something, anything, to make it pop! Now we're doing graphic design. Now there's almost no searching left. Without any text someone could look at your piece and immediately know to focus on the bottle. Maybe you differentiate the entire person with the bottle well thats okay, you still have the text to further support it.
The idea is finding a balance. How many iterations it takes is up to you. But with any design of anything you have to either really look at it as though its your first time or get others to test it for you. Once someone knows the product then they have expectations. They're looking for your product already. You want to make your design communicate what that product is as well as you can allowing end-users to understand, and remember it, with as little bottom-up processing as possible. The less they have to scan before understanding, the better.
(I'll redo this example with pictures over the next few days)
Here is a great paper on Top-Down Processing that Transitions into those very principles: What's up in top-down processing? by Cavanagh, Patrick.
Here's a wonderful article that came out in The Atlantic today (May 5, 2014) which stays on fairly abstract terms non-designers can understand but does clearly state: Things You Cannot Unsee (And What That Says About Your Brain)
In short: what you know influences what you see.