added getConfidence() with comment

java/com.sap.sailing.domain/src/com/sap/sailing/domain/base/HasConfidence.java

@@ -13,5 +13,28 @@ package com.sap.sailing.domain.base;
  *
  */
 public interface HasConfidence {
-
+    /**
+     * A confidence is a number between 0.0 and 1.0 (inclusive) where 0.0 means that the value is randomly guessed while
+     * 1.0 means the value is authoritatively known for a fact. It represents the weight with which a value is to be
+     * considered by averaging, interpolation and extrapolation algorithms.
+     * <p>
+     * 
+     * An averaging algorithm for a sequence of <code>n</code> tuples <code>(v1, c1), ..., (vn, cn)</code> of a value
+     * <code>vi</code> with a confidence <code>ci</code> each can for example look like this:
+     * <code>a := (c1*v1 + ... + cn*vn) / (c1 + ... + cn)</code>. For a single value with a confidence this trivially
+     * results in <code>c1*v1 / (c1)</code> which is equivalent to <code>v1</code>. As another example, consider two
+     * values with equal confidence <code>0.8</code>. Then, <code>a := (0.8*v1 + 0.8*vn) / (0.8 + 0.8)</code> which
+     * resolves to <code>0.5*v1 + 0.5*v2</code> which is obviously the arithmetic mean of the two values. If one value
+     * has confidence <code>0.8</code> and the other <code>0.4</code>, then
+     * <code>a := (0.8*v1 + 0.4*vn) / (0.8 + 0.4)</code> which resolves to <code>2/3*v1 + 1/3*v2</code> which is a
+     * weighed average.
+     * <p>
+     * 
+     * Note, that this doesn't exactly take facts for facts. In other words, if one value is provided with a confidence
+     * of <code>1.0</code>, the average may still be influenced by other values. However, this cleanly resolves otherwise
+     * mutually contradictory "facts" such a <code>(v1, 1.0), (v2, 1.0)</code> with <code>v1 != v2</code>. It is
+     * considered bad practice to claim a fact as soon as it results from any kind of measurement or estimation. All
+     * measurement devices produce some statistical errors, no matter how small (cf. Heisenberg ;-) ).
+     */
+    double getConfidence();
 }