Why a drop in traffic on my website?

My site experiences a sudden drop in visits from search engines. How do I know what causes it?

The loss of traffic to a site can have various causes and some may be unsuspected, unexpected. This does not necessarily com from the design or the content of the site and may be completely out of reach of the webmaster.

Internal causes

Metadata

Several types of information in the pages can block any traffic to a site:

Optimization criteria

If they have not been changed, there is no reason that traffic diminish, but a change may trigger a negative signal for the search engine algorithm.

Penalties

An external effect may have an internal cause. A site may be penalized by search engines for competitive keywords if it introduced in its pages signals of what is considered as spam.
Here are a few:

Penalties consist of a loss of position in the results pages ranging from a few lines to several hundred pages.

External causes

Changes in the algorithm

We saw at the MayDay, a significant change in Google's algorithm can reduce the traffic of a site by 70%, which certainly benefits to other sites.
MayDay, who was given this name because it affected most sites May 1, 2010, affected the long tail, the requests of several keywords that are individually uncommon but together make up the bulk of traffic .
It has been redirected to pages with more content.

The same effect was observed after the Panda Update, which penalizes sites whose pages do not have enough original content, and the Penguin Update, which takes to sites with too many self-made back links​​, so from sources without any authority in the field.

Google suggest

This is something that we do not think initially, but the search suggestions, adding one or two keywords to what the user types, have the effect of directing it to sites that contain these groups of keywords. Suggest may also evolve, be refined, provide different suggestions.
That's why some webmasters have seen their traffic that took advantage of these keywords for over a year to drop from 5000 UV per day to almost nothing, the day where a group of keywords has been deleted.
To this, you can not do anything.

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