ImageA Nobel Prize medal in 2020.Credit…Jacquelyn Martin/Associated PressDavid Card, Joshua D. Angrist and Guido W. Imbens have made a career of studying unintended experiments — Mr. Card in labor economics and Mr. Angrist and Mr. Imbens in analyzing cause and effect.On Monday, their work earned them the 2021 Nobel Memorial Prize in Economic Sciences.All three winners are based in the United States. Mr. Card, who was born in Canada, works at the University of California, Berkeley. Mr. Angrist, born in the United States, is at M.I.T. and Mr. Imbens, born in the Netherlands, is at Stanford University.“Uncovering causal relationships is a major challenge,” said Peter Fredriksson, chairman of the prize committee. “Sometimes, nature, or policy changes, provide situations that resemble randomized experiments. This year’s laureates have shown that such natural experiments help answer important questions for society.” Mr. Card’s work has challenged conventional wisdom in labor economics — including the idea that higher minimum wages led to lower employment. He was a co-author of influential studies on that topic with Alan Krueger, who died in 2019, including one that used the natural geographical border between New Jersey and Pennsylvania to test the effect of a minimum wage change. Comparing outcomes between the states, the research found that employment at fast food restaurants was not affected by an increase in New Jersey’s minimum wage. Mr. Angrist and Mr. Imbens have contributed to the way researchers think about and analyze natural experiments. Two American economists affiliated with Stanford University, Paul R. Milgrom and Robert B. Wilson, won the 2020 Nobel in economics for improvements to auction theory. Abhijit Banerjee and Esther Duflo of M.I.T. and Michael Kremer of Harvard won in 2019 for their experiment-based research in development economics.The award, formally called the Sveriges Riksbank Prize in Economic Sciences in Memory of Alfred Nobel, has been given 53 times since 1969.Who are the other Nobel Prize winners in 2021?The Nobel Prize in Physiology or Medicine was awarded jointly to David Julius and Ardem Patapoutian, who independently discovered key mechanisms of how people sense heat, cold, touch and their own bodily movements.Syukuro Manabe, Klaus Hasselmann and Giorgio Parisi were recognized with the Nobel Prize in Physics for work that “laid the foundation of our knowledge of the Earth’s climate and how humanity influences it.”The Nobel Prize in Chemistry went to Benjamin List and David W.C. MacMillan for their development of a new tool to build molecules, work that has spurred advances in pharmaceutical research and allowed scientists to construct catalysts with considerably less impact on the environment.Abdulrazak Gurnah was awarded the Nobel Prize in Literature for “his uncompromising and compassionate penetration of the effects of colonialism and the fate of the refugee in the gulf between cultures and continents.”Two journalists thousands of miles apart, Maria Ressa of the Philippines and Dmitri A. Muratov of Russia, were honored with the Nobel Peace Prize for their tireless efforts to hold the powerful to account.Read morePassengers line up at a Southwest Airlines ticket counter at Eppley Airfield in Omaha on Sunday. The airline cancelled over 1,800 flights over the weekend, and said weather conditions and air traffic control issues were to blame.Credit…David Zalubowski/Associated PressTravelers flying Southwest Airlines over the weekend faced hundreds of canceled flights as well as frustrating delays, with the airline scrapping about a quarter of all its flights on Saturday and Sunday.The airline canceled just over 800 flights on Saturday, or 24 percent of all its scheduled flights, according to FlightAware, a tracking service. By noon on Sunday, Southwest had already canceled over a thousand flights, or 28 percent of its schedule, with hundreds more delayed.“We experienced weather challenges in our Florida airports at the beginning of the weekend, challenges that were compounded by unexpected air traffic control issues in the same region, triggering delays and prompting significant cancellations,” the airline said in a statement on Sunday. “We’ve continued diligent work throughout the weekend to reset our operation with a focus on getting aircraft and crews repositioned to take care of our customers.”Southwest added that recovering from the disruption was more difficult than usual because it is operating fewer flights than before the pandemic, complicating efforts to reschedule passengers.