The stable isotope composition of foraminifera is routinely measured using a pool of shells from a single species, this pooling has been referred to as conventional isotope analysis (CIA) by Ganssen et al. (2011). The results of monospecific pools of foraminifera are used by Palaeoclimatologists and Palaeoceanographers to infer the temperature of the past, global ice volume or as a stratigraphic tool for determining the age of ocean sediments. Pooling numerous shells gives an average value, although this may not reflect a climatological average (Mix et al., 1987; Roche et al., 2018; see Data-model comparison). Single specimen analysis also referred to as individual specimen analysis (ISA) or individual foraminiferal analysis (IFA) is oxygen or carbon isotope (δ18O; δ13C) analysis that enables a better understanding of the variability within the foraminiferal population. The variability identified within samples was shown by the pioneers almost 40 years ago (a quick 'recap' can be found in Ganssen et al.) in a paper entitled "Oxygen and carbon isotopes of individual shells of planktonic foraminifera from Ontong-Java plateau, equatorial pacific" (Killingley et al., 1981). In this paper Killingley et al. (1981) measured individuals of Orbulina universa with a size between 900 and 1100 μm to ensure that a mass of 100 μg was reached, this size range appears excessive, however it was a necessary requirement to gain sufficient sample gas in order to for the measured values to have a degree of precision. Early mass spectrometers used in palaeo-research required a large sample size (equivalent of 4.5 mg carbonate) that was slowly reduced (Shackleton et al. 1965) with modifications (Breitenbach et al., 2011; Fiebig et al., 2005; Ishimura et al., 2008) until most modern mass spectrometers (Ganssen et al., 2005; Ganssen et al., 2011; Koutavas et al., 2006a ; Niitsuma et al., 1991; Spero and Williams, 1990; Stott and Tang, 1996; Tang and Stott, 1993) have become capable of measuring single foraminifera of small mass or even individual chambers (Lougheed et al., 2018; Pracht et al., 2019; Takagi et al., 2015; Takagi et al., 2016).

Following on from this pioneering study(Killingley et al., 1981), numerous workers have used individual foraminiferal δ18O analysis of modern foraminifera to determine the depth habitat of species (Oba, 1990; 1991; Takagi et al., 2015; Takagi et al., 2016; Steinhardt et al., 2015); seasonality (Billups and Spero, 1996; Ganssen et al., 2011; Groenveld et al., 2019; Metcalfe et al., 2019; Naidu et al., 2019; Stott and Tang, 1996; Wit et al., 2010); ecology (Metcalfe et al., 2015; Mikis et al., 2019; Feldmeijer et al., 2015; Pracht et al., 2019; Sadekov et al., 2016); the occurrence of low salinity events (Spero and Williams, 1990; Tang and Stott, 1993; Vetter et al., 2017); ENSO dynamics (Koutavas et al., 2006a; 2006b; Koutavas and Joanides, 2012; Leduc et al., 2009; Rustic et al., 2015); expatriation (Scussolini et al., 2013; van Sebille et al., 2015); and bioturbation (Lougheed et al., 2018; Stott and Tang, 1996; Wit et al., 2013). Such data has also helped in modelling of samples or determining sample uncertainty (Schiffelbein and Hills, 1984; Schiffelbein 1985; 1986; Wit et al., 2013), with analytical tools including: the Individual Foraminiferal approach uncertainty analysis - INFAUNAL (Thirumalai et al., 2013); the Foraminiferal Isotope Reproducibility Model - FIRM (Fraass and Lowery, 2017); and Sedproxy (Dolman and Laepple, 2018). Therefore, through the use of individual foraminiferal analysis it is possible to not only better understand the climate system but to (critically) assess our proxy archives.