Question: While Kurt Kren is a film maker, John Grierson is a what?

Context paragraphs:
Michael Darlow: Michael Darlow (born 13 June 1934) is a British television producer, director and writer.  After starting as an actor, his first short film was seen by documentary film maker John Grierson and shown on TV and at the 1960 Edinburgh Film Festival.  Darlow's documentary, drama and arts programmes have won international awards including BAFTAs, an Emmy, and Gold at the New York Television Festival.  His works include The World At War episode "Genocide", The Barretts of Wimpole Street, Johnny Cash at San Quentin and "Bomber Harris".  He is a Fellow of the Royal Television Society.

John Grierson: John Grierson CBE (26 April 1898 – 19 February 1972) was a pioneering Scottish documentary maker, often considered the father of British and Canadian documentary film.  In 1926, Grierson coined the term "documentary" in a review of Robert Flaherty's "Moana".

Harry Watt: Harry Watt (18 October 19062 April 1987) was a Scottish documentary and feature film director, who began his career working for John Grierson and Robert Flaherty.  His 1959 film "The Siege of Pinchgut" was entered into the 9th Berlin International Film Festival.

Kurt Kren: Kurt Kren (September 20, 1929 - died in Vienna on June 23, 1998) was an Austrian avant-garde filmmaker.  He is best known for his involvement with the Vienna Aktionists and the group of films that resulted, although this accounts for only a part of his career, and he later returned to the Structural roots of his third film "3/60: Bäume Im Herbst".  Although not a seminal Structural film, "31/75: Asyl" is arguably one of the more satisfying films of the movement.

Night Mail: Night Mail is a 1936 documentary film about a London, Midland and Scottish Railway (LMS) mail train from London to Scotland, produced by the GPO Film Unit.  The film ends with a "verse commentary" by W. H. Auden, written for existing footage.  Benjamin Britten scored the film.  The film was directed by Harry Watt and Basil Wright, and narrated by John Grierson and Stuart Legg.  The Brazilian filmmaker Alberto Cavalcanti was sound director.  The locomotive featured in the film was Royal Scot 6115 "Scots Guardsman", built in 1927.  The film has become a classic of its own kind, much imitated by adverts and modern film shorts.  "Night Mail" is widely considered a masterpiece of the British Documentary Film Movement.

Grierson Awards: The Grierson Awards celebrate innovative and exciting new documentary films.  The awards have been set up by the Grierson Trust to commemorate the life and work of world-renowned documentary filmmaker John Grierson.  The awards were first set up in 1972 and have run annually.  In 2000 the Grierson Trust forged a link with the UK Film Council in order to expand the awards and add more prestige to the awards.  The awards have grown in stature and recognition over the years and now are "more important (than ever).  They have an impressive list engaging with a broad palette of styles and subjects from disabled people looking for love to asylums, Russian billionaires and global warming."

John Grierson (Dominican): John Grierson or Grisson (died 1564?)  was a Scottish Dominican and perhaps a member of the family of Grierson of Lag in Dumfriesshire.

Drifters (1929 film): Drifters (1929) is silent documentary film by John Grierson, his first and only "personal film".

Documentary News Letter: Documentary News Letter was a magazine about documentary film founded by filmmaker John Grierson.  The publication was developed as the wartime successor to "World Film News", which ceased publication in 1938, and sought to promote a "documentary approach to everyday living."  It published its first edition in January 1940, with an editorial board that consisted of Grierson, Paul Rotha, Basil Wright, Arthur Elton and Thomas Baird.

GPO Film Unit: The GPO Film Unit was a subdivision of the UK General Post Office.  The unit was established in 1933, taking on responsibilities of the Empire Marketing Board Film Unit.  Headed by John Grierson, it was set up to produce sponsored documentary films mainly related to the activities of the GPO.

Supporting paragraphs: Kurt Kren John Grierson
Answer: documentary maker