The Future Circular Collider study complements previous studies for linear colliders. Colliders may either be ring accelerators or linear accelerators. Squarks can be produced through strong interactions and therefore are easily produced at hadron colliders. The concept of missing energy is commonly applied in hadron colliders. Only a few hadron colliders have been built. High energy particle colliders have been used to try to create magnetic monopoles. Some of these lepton colliders are still running. It aims to complement existing technical designs for linear electron/positron colliders (ILC and CLIC). The Bhabha scattering rate is used as a luminosity monitor in electron-positron colliders. It is important in experimental and theoretical studies of interactions at particle colliders. Unlike colliders, confounder variables "should" be controlled for when estimating causal associations. Colliders are sometimes confused with confounder variables. Colliders can therefore undermine attempts to test causal theories. There are other signatures of large extra dimensions at high energy colliders. Historically, the tightest limits were from direct production at colliders.