What factors impact accuracy?

What are the most important factors influencing a mechanical watches accuracy?
Mechanical watches are generally either automatic or hand wound but there are many factors which will affect the accuracy. The most important of these are environmental factors such as temperature, gravity, vibration and shock. Other factors such as physical activity and the way in which the watch is worn can mean that the accuracy will vary depending on whether you are very active or purely sitting at a desk.

Temperature : Temperature is also very relevant to accuracy for example 
Keep in mind that if you are lying in the sun and the heat causes is the case to become extremely hot accuracy will be affected and it will also be impacted by extremely cold conditions. Another factor to keep in mind is the combination of the two because frequently people are lying in the sun and will dive into a cold pool which can cause thermal shock to the watch.

Although temperature is unpredictable we generally find that with high temperatures, a watch tends to lose time, and with low temperatures, It's more likely to run fast.

Shock and Vibration : Shock and vibration can greatly affect accuracy
Most manufacturers recommend that you remove or watch before taking part in any activities which involve significant impact. One of the worst things for a watch is anything involving firearms, power tools or intense vibration this is  because the hands can be dislodged  even activities such as golf can have a negative effect. In the case of military watches and those made for Police most manufacturers take steps to attempt to protect the watches as much as possible from the effects of shock from firearms for obvious reasons but this is not entirely effective because the movement Itself has inherent weaknesses which cannot be overcome.

The most likely location for watches to be damaged is normally in the bathroom if they dropped onto a tiled surface or if they are dropped in the kitchen any significant impact can damage the movement, crack the crystal or dislodge the hands.

The problem of magnetism and magnetic fields
Magnetism can be a major headache with hand wound and automatic watches, the most commonly affected component is the hairspring, even though most of the mechanical movements which we use would tend to use Nivarox (Nivarox is an acronym for Nicht Variabel Oxydfest (in English Non-Variable Oxidizing Resistant) although these hairsprings contain modern alloys that are much less affected by magnetism and are supposed to be highly resistant to magnetic fields in real life magnetism can still be a problem because the alloy is a combination of iron and nickel, even though the mechanical movements we use are either Seiko or sometimes Sellita the spring can still suffer issues. In simple terms what happens is that the coils of the hairspring stick together, this means is that the spring is actually then somewhat shorter in real terms which consequently causes the watch to gain. Although fortunately it's not particularly common it's not unheard of for the springs to stick together to such a degree that the watch movement cannot operate at all.

Fortunately no damage is normally done and the watch can be demagnetised by any watchmaker who should be able to do the job in seconds although as it has a risk of re-occurring it often makes sense to simply buy a demagnetiser. Rarely it needs to be performed twice but usually once is enough, the main thing is the watch doesn't come into contact again with whatever caused the problem in the first place.

The basic problem is that magnets are everywhere, examples are the scanners at airports, MRI machines smartphones and tablets, handbags, which frequently have magnets in their clasps and worst of all items such as speakers and TVs, even the door contacts of a security system on Windows are magnetic in many cases. The problem is because magnets and magnetism is so prevalent sometimes you wouldn't even realise that you've come into contact.

If you have a compass it's easy to find out if this problem has arisen, just place your watch near a compass. If the compass needle moves, your watch has been magnetised. It's a 100% reliable diagnosis.

Position : The accuracy of the watch is also affected by the position in which is placed when it is not being worn
Although not significant to the same degree as some other factors a watch will either lose or gain time depending on the position in which its placed when not being worn. Whether it's placed on its side, face or caseback will usually affect the degree to which the watch gains or loses. This means its possible to regulate a watch when it's off the wrist, for example at night because if you know that when it's being worn it’s prone to gain a few seconds each day you may find a specific position will have the opposite effect which means that the watch will not need to be adjusted as frequently because it can simply be achieved by carefully positioning the watch each evening before you go to bed in the position that has the desired effect on its timekeeping.

The reason that carefully positioning a watch works this way is because gravity impacts the operation of the movement hence even the slightest change can amount to a few seconds over seven or eight hours because with 86,400 seconds per day even if a watch is 99.9% accurate, it will still be off by a minute and a half in only 24 hours! For this reason even the slightest change to its timekeeping can make a worthwhile change. The key is knowing which particular position yields the required result.