Just how do you know when you are writing clearly and simply? You don't, not unless you have a guide, and there are some good ones available.  But before I give you a

 readability guide for your writing, let's consider what people prefer to read.

Probably the most universally read magazine in the United States today is the Reader's Digest.  Millions devour it every month, and almost everyone can understand it easily.  McCalls and Newsweek are also read by an extremely large audience.  They are a little more difficult to read than Reader's Digest, but certainly clear and understandable.  But as we move toward the Atlantic monthly and Harpers Magazine, we begin to lose our audience.  These last two are considered primarily highbrow and almost all of their readers are college trained and intellectually inclined.  Beyond this, we find technical writing, governmental information, textbooks, and similar material which very few readers will even try to read unless it's absolutely necessary.

Obviously, then to reach your readers effectively, you should stay somewhere in the middle range of the Reader's Digest, McCalls, Newsweek and similar magazines.  The question is, of course, just what is this middle range.  There are a number readability guides the one developed by Rudolf Flesch and Robert Gunning are probably two of the best, but I find both a little cumbersome to use so I will outline one here, which can be applied easy to your writing

(1.) Count the number of sentences in a sample of a hundred words and divide the number of sentences by one hundred. (This is the average number of words per sentence.)  2.  Count all the words over seven letters in length. (3.) Add both figures together and divide the sum by two.  This gives you a workable readability index.  For instance, a one hundred word passage with six sentences, six words with over seven letters each adds up this way: 100/6 equals 16.  16 plus the six equals 22.  Divide this by two and you have a readability index of 11.  Here's how some well-known reading material tests out:

Children's writing 10 -- 12.

General magazines, 13 -- 15.

Literary magazines, 17 -- 20.

Most textbooks, technical materials, 23 plus.

For good readability then you should try to keep your own writing somewhere between 12 and 16.  Several studies have shown that employees that communicate best are those that do to the better job and are promoted quickly.  In addition, people who keep their writing in this range get more action, better understanding from others, and overall favorable response.

Keep Sentence Length in Check

People read in sentences, not in words.  Research over the last 20 years to determine the effect of sentence length on readability has shown that the longer a sentence is, the harder it is to read.

Here is a gauge:

Very Easy                    8 words or less per sentence.

Easy                            9 -- 11 words

Fairly Easy                  12 -- 14 words.

Average                       15 -- 19 words.

Hard                            25 -- 28 words.

Very Hard                    29 words are more.

This doesn't mean that every sentence you write should be 15 to 19 words long, that would be boring.  Mix it up, have a few 8 word sentences, some 25 word sentences, some 15, 19 word sentences, and a few in between.  Overall, however, make sure your sentences average in the fairly easy to average range.  This will ensure your writing will be read.