How many Jews in Siedlce in 1939

The dilemma - population of  Siedlce in 1939 

Demography calculation depends on many factors like any statistics ... if used out of context may become not accurate.
Typically genealogists are concerned with demography ... putting it to test for Siedlce is an interesting case study.  Today with the abundance of information readily obtainable over the Internet it should be a "piece of cake" task.
Well, it does't.  Once a person  began to check how many Jews lived in Siedlce in 1939 just before the war it appears not so trivial.

The bottom line seems to be this:

Total Population      41,294

Jewish Population   15,823

Percentage               38.3%

The following article is short enough and interesting for the readers not only as a source for demography figures so it appears in full.

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March 13, 2009 - Commemorating of the Jewish inhabitants of Siedlce
 Before World War II Jews constituted 70 % of the city’s population. Today they have been commemorated with a plaque that was founded by the Foundation for the Preservation of Jewish Heritage and the Jewish Community of Warsaw.
Apart from the Siedlce residents and city authorities’ representatives, students of the classes 1-3 of the Bolesław Prus High School, a group of almost one hundred girls from Israel and the USA, employees of the Podlasie Academy, as well as a Slovak whose entire family died during the Holocaust took part in the ceremony. Pinchas Żarczyński, the assistant rabbi of the Warsaw community said a prayer for the deceased – Yizkor and the vice-mayor of Siedlce Jarosław Głowacki, together with other city’s deputies, laid flowers at the monument commemorating the two thousand murdered Jews.
A US lawyer, Michael Traison additionally sponsored an iron signpost with a huge Star of David, informing about the Jewish cemetery. It was located at the junction of Szkolna and Piłsudski streets, 300m away from the cemetery.
References:
„Tablica ku pamięci Żydów siedleckich”, [in:] „Życie Siedleckie”, 06.03.2009;
„Uroczystość na siedleckim cmentarzu żydowskim”, [in:] „ Tygodnik Siedlecki”, 22.03.2009; 
„Tablica, drogowskaz i cała reszta”, [in:] „Tygodnik Siedlecki”, 05.04.2009.
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Before the war means a period of 20 years 1919-11930. So let's drill it it down to the smaller chunks of chronology.

Wikipedia adds some more insights: Here the population of Jews is 15,000 or 37%.  

"For much of the 19th Century - a time when the town's population steadily increased - Jews were the majority of Siedlce's population: 3,727 (71.5%) in 1839; 4,359 (65%) in 1841; 5,153 (67.5%) in 1858; 8,156 (64%) in 1878. 

Later on, the percentage of Jews decreased due to non-Jewish migration: according to Russian census of 1897, out of the total population of 23,700, Jews constituted 11,400 (so around 48% percent).^[1] 

The first Polish census, in 1921, recorded 14,685 Jews living in Siedlce. 

Their number remained steady in the interwar period, and in 1939, on the eve of the Second World War, there were some 15,000 Jews living in the town.^{[citation needed]}

"In 1939, Jews constituted some 37% of the town's population." 

The above Wikipedia figures are consistent with Jewish Virtual Library: 

"The Jewish population numbered 3,727 (71.5% of the total) in 1839; 4,359 (65%) in 1841; 5,153 (67.5%) in 1858:8,156 (64%) in 1878; and 14,685 (47.9%) in 1921."

Another even richer source is Enote on Siedlce ... info here like above.

Encyclopedia of Camps and Ghettos (page 428) provides slightly different figures; 17,000 Jews - as per the Encyclopedia it was 50% of the population on the eve of WWII. 

As per Polish Version of Wikipedia -  ... as in 1939 the population of Siedlce was 41,294 (this figure is also confirmed here (refer to Ludność Siedlec); according to the figure from "Zydzi w Siedlcach 1850-1945)" Table 4 page 230 - there were in total 15,823 Jews above it gives us 38.3% of Jewish population.