user14433: I want to detect if a seat is occupied by a person. This is the answer that I found on Stackoverflow. 
However, how accurate is the output of 4 suggested sensors? Will the computing of all these 4 sensor values into one boolean recognize a case when, for instance, a box of 40kg is placed on a seat and some person is just slightly "shaking" that seat with a hand? In those cases Arduino should recognize that a seat is not occupied by a person. Will it work with only 4 sensors or is there any better and more accurate solution (maybe some existing library)? I guess that CapSense-Library is not applicable in my case.

user4143: As has been mentioned in the comments there's many ways of sensing, each with its own pros and cons.
Sensing the presence of something is actually twofold.  First you need to know if there is something there, and secondly you need to try and identify what it is that's there and if it fits your criteria.
With passive sensors of the type used by Arduinos it all comes down to a matter of probability.
If you have force sensors, you know when something is there, but you can't work out what is there, only that it weighs a certain amount.
Using capacitative sensors you can get an idea about what the something is made of - how much does it affect the capacitance, etc.
Using ultrasound and/or infra-red beams etc you can get an idea of the kind of size of the object that is in the chair.
So combining them all together you can work out the likelihood that it is a human if:

It weighs enough to be a human
It has a suitable dielectric response to be a human
It is big enough to be a human

Of course, there's always the chance that it's just a big dog, or a sack of potatoes.
So it's all a case of improving that probability by adding more sensors of different types, until you are almost certain that it is a human.  You will never be 100% certain, of course.

14433: <p>I want to detect if a seat is occupied by a person. This is <a href="https://arduino.stackexchange.com/questions/1288/detect-and-stream-if-a-chair-is-occupied/1308#1308">the answer</a> that I found on Stackoverflow. </p>

<p>However, how accurate is the output of 4 suggested sensors? Will the computing of all these 4 sensor values into one boolean recognize a case when, for instance, a box of 40kg is placed on a seat and some person is just slightly "shaking" that seat with a hand? In those cases Arduino should recognize that a seat is not occupied by a person. Will it work with only 4 sensors or is there any better and more accurate solution (maybe some existing library)? I guess that CapSense-Library is not applicable in my case.</p>
 4143: <p>As has been mentioned in the comments there's many ways of sensing, each with its own pros and cons.</p>

<p>Sensing the presence of something is actually twofold.  First you need to know if there is something there, and secondly you need to try and identify <em>what</em> it is that's there and if it fits your criteria.</p>

<p>With passive sensors of the type used by Arduinos it all comes down to a matter of <em>probability</em>.</p>

<p>If you have force sensors, you know when <em>something</em> is there, but you can't work out <em>what</em> is there, only that it weighs a certain amount.</p>

<p>Using capacitative sensors you can get an idea about what the <em>something</em> is made of - how much does it affect the capacitance, etc.</p>

<p>Using ultrasound and/or infra-red beams etc you can get an idea of the kind of size of the object that is in the chair.</p>

<p>So combining them all together you can work out the likelihood that it is a human if:</p>

<ul>
<li>It weighs enough to be a human</li>
<li>It has a suitable dielectric response to be a human</li>
<li>It is big enough to be a human</li>
</ul>

<p>Of course, there's always the chance that it's just a big dog, or a sack of potatoes.</p>

<p>So it's all a case of improving that probability by adding more sensors of different types, until you are <em>almost</em> certain that it is a human.  You will never be 100% certain, of course.</p>
