Research in positive psychology, well-being, eudaimonia and happiness, and the theories of Diener, Ryff, Keyes and Seligman cover a broad range of topics including "the biological, personal, relational, institutional, cultural, and global dimensions of life".  A meta-analysis on 49 studies in 2009 showed that Positive Psychology Interventions (PPI) produced improvements in well-being and lower depression levels, the PPIs studied included writing gratitude letters, learning optimistic thinking, replaying positive life experiences and socializing with others.  In a later meta-analysis of 39 studies with 6,139 participants in 2012, the outcomes were positive. Three to six months after a PPI the effects for subjective well-being and psychological well-being were still significant. However the positive effect was weaker than in the 2009 meta analysis, the authors concluded that this was because they only used higher quality studies. The PPIs they considered included counting blessings, kindness practices, making personal goals, showing gratitude and focusing on personal strengths.