"some studies observed" is correct and usable in written English.
You can use it when you are referring to research or studies that have already been done, and that have already made some observations. For example, "Some studies observed that participants who took part in the trial showed significant improvements in their physical health."
However, some studies observed that gestational hypertension in twin pregnancy could be beneficial for foetal growth.
Compared to DON, some studies observed higher cytotoxicity for NIV (Minervini et al. 2004).
Some studies observed that throat microbiota did not reflect the microbiota of the lower airways in some CF patients [10, 56, 61].
However there is still controversy if the relationship between maternal age at delivery and ALL is linear or u-shaped as some studies observed an increased risk of ALL in children of mothers aged <20 years [2], [4], [12], [23].
However, some studies observed different results.
In some studies observed counts may be zero.
Some studies observe the frequency and causes of workarounds; others examine attitudes of professionals to circumvention of rules.
Studies assessing the rewarding effects of cocaine in 129 substrains are more variable with some studies observing no place preference [ 28] and others observing significant place preference [ 27].
Some studies observe higher erythrocyte TBARS levels in AD [ 5, 7, 31], while others observe no difference between AD and controls [ 6, 15, 29, 36].
In particular, some studies, observing that ovarian CCC characterized by HNF1B overexpression were often chemo-resistant, suggested a possible effect of the level of HNF1B on chemosensitivity.
Studies have also been conducted into β-cell autoimmunity and breast-feeding, but the results have been mixed, with some studies observing protective effects of breast-feeding, whereas others have observed no association [see Knip et al. (15) for references].
